Data source and key concepts

National Hospital Morbidity Database

Data are sourced from the National Hospital Morbidity Database (NHMD); a collation of data on admitted patient care in Australian hospitals defined by the Admitted Patient Care National Minimum Data Set (APC NMDS). It is possible for patients to have multiple hospitalisations in any given reference period. The statistical measures presented are derived based on episodes of care that ended within a collection period. The NHMD is a compilation of episode-level records from admitted patient morbidity data collections in Australian hospitals. It includes demographic, administrative and length of stay data for each hospitalisation. Clinical information such as diagnoses, procedures undergone and external causes of injury and poisoning are also recorded. For further details on the scope and quality of data in the NHMD, refer to Admitted patient care: Australian Hospital Statistics 2019–20.

The 2019–20 collection contains data for separations (referred to as hospitalisations in this report) that occurred between 1 July 2019 and 30 June 2020. Admitted patient episodes of care/hospitalisations that began before 1 July 2019 are included if the hospitalisation date fell within the collection period (2019–20). A record is generated for each hospitalisation rather than each patient. Therefore, those patients who separated from hospital more than once in the reference year have more than one record in the database.

Specialised mental health care is identified by the patient having 1 or more psychiatric care days recorded—that is, care was received in a specialised psychiatric unit or ward during that hospitalisation. In public acute hospitals, a ‘specialised’ episode of care or hospitalisation may comprise some psychiatric care days and some days in general care. An episode of care from a public psychiatric hospital is deemed to comprise psychiatric care days only and to be ‘specialised’, unless some care was given in a unit other than a psychiatric unit, such as a drug and alcohol unit.

Although there are national standards for data on admitted patient care, the results presented here may be affected by variations in admission and reporting practices between states and territories. Interpretation of the differences between states and territories therefore needs to be made with care. The principal diagnosis refers to the diagnosis established after observation by medical staff to be chiefly responsible for the patient’s episode of admitted patient care. For 2019–20, diagnoses are classified according to the International Statistical Classification of Diseases and Related Health Problems, 11th revision, Australian Modification (ICD‑10‑AM 11th edition) (ACCD 2016). Further information on this is included in the technical information section.

For 2019–20, procedures are classified according to the Australian Classification of Health Interventions, 10th edition. Further information on this classification is included in the technical information section. More than 1 procedure can be reported for a hospitalisation and not all hospitalisations have a procedure reported.

The large decline in patient days associated with public hospital mental health-related hospitalisations from 2016–17 to 2017–18 followed large increases from 2014–15 to 2016–17. These fluctuations are likely to be related to the introduction of the Mental health care type from 1 July 2015. For example, to change the care type of patients receiving mental health care, Queensland (in 2015–16) and New South Wales (in 2016–17) discharged and re-admitted patients, causing the rise in hospitalisations and patient days counted in those years. The rise in patient days is substantially impacted by long stay mental health patients, who can individually account for hundreds, or in some cases thousands, of days. The subsequent decline in patient days seen in 2017–18 is impacted by days accrued before the change in care type being counted in an earlier year.

Key concepts

Overnight admitted mental health-related care
Key concept Description
Average length of stay Average length of stay is the average number of patient days for admitted patient hospitalisations.
Care type

The care type defines the overall nature of a clinical service provided to an admitted patient during an episode of care (admitted care), or the type of service provided by the hospital for boarders or posthumous organ procurement (other care).


Hospitalisation is the term used to refer to the episode of admitted patient care, which can be a total hospital stay (from admission to discharge, transfer or death) or a portion of a hospital stay beginning or ending in a change of type of care (for example, from acute care to rehabilitation).

Mental health-related hospitalisation

A hospitalisation is classified as mental health-related for the purposes of this report if:

  • it had a mental health-related principal diagnosis, which, for admitted patient care in this report, is defined as a principal diagnosis that is either:
    • a diagnosis that falls within the section on Mental and behavioural disorders (Chapter 5) in the International Statistical Classification of Diseases and Related Health Problems, 10th revision, Australian Modification (ICD‑10‑AM) (codes F00–F99), or
    • a number of other selected diagnoses (see the technical information) for a full list of applicable diagnoses), and/or
  • it included any specialised psychiatric care.
Overnight admitted patient care

For this report overnight admitted patient hospitalisations refers to those hospitalisations when a patient undergoes a hospital’s formal admission process, completes an episode of care, is in hospital for more than one day and ‘separates’ from the hospital. Same-day hospitalisations are reported separately in the Admitted patient care – same-day care section of this report.

Patient day

Patient day means the occupancy of a hospital bed (or chair in the case of some same day patients) by an admitted patient for all or part of a day. The length of stay for an overnight patient is calculated by subtracting the date the patient was admitted from the date of separation and deducting days the patient was on leave. A same-day patient is allocated a length of stay of 1 day. Patient day statistics can be used to provide information on hospital activity that, unlike hospitalisation statistics, account for differences in length of stay. The patient day data presented in this report include days within hospital stays that occurred before 1 July provided that the hospitalisation occurred during the relevant reporting period (that is, the financial year period). This has little or no impact in private and public acute hospitals, where hospitalisations are relatively brief, the amount of information delivered is relatively high and the patient days that occurred in the previous year are expected to be approximately balanced by the patient days not included in the counts because they are associated with patients yet to separate from the hospital and therefore yet to be reported. However, some public psychiatric hospitals provide very long stays for a small number of patients and, as a result, would have comparatively large numbers of patient days recorded that occurred before the relevant reporting period and may not be balanced by patient days associated with patients yet to separate from the hospital.

Principal diagnosis

The principal diagnosis is the diagnosis established after study to be chiefly responsible for occasioning the patient’s episode of admitted patient care.


Procedure refers to a clinical intervention that is surgical in nature, carries an anaesthetic risk, requires specialised training and/or requires special facilities or services available only in an acute care setting. Procedures therefore encompass surgical procedures and non-surgical investigative and therapeutic procedures, such as X-rays. Patient support interventions that are neither investigative nor therapeutic (such as anaesthesia) are also included.

Procedures are grouped together in blocks (Procedure blocks) based on the area of the body, health professional or intervention involved.

Psychiatric care days

Psychiatric care days are the number of days or part days the person received care as an admitted patient in a designated psychiatric unit or ward.


Separation means the process by which an admitted patient completes an episode of care by being discharged, dying, transferring to another hospital or changing type of care. Each record includes information on patient length of stay. A same-day separation occurs when a patient is admitted and separated from the hospital on the same date. An overnight separation occurs when a patient is admitted to and separated from the hospital on different dates. The numbers of separations and patient days can be a less reliable measure of the activity for establishments such as public psychiatric hospitals, and for patients receiving care other than acute care, for which more variable lengths of stay are reported.

Specialised psychiatric care

A hospitalisation is classified as having specialised psychiatric care if the patient was reported as having one or more days in a specialised psychiatric unit or ward.

Without specialised psychiatric care

A hospitalisation is classified as without specialised psychiatric care if the patient did not receive any days of care in a specialised psychiatric unit or ward. Despite this, these hospitalisations are classified as mental health related because the reported principal diagnosis for the hospitalisation is either one that falls within the Mental and behavioural disorders chapter (Chapter 5) in the ICD‑10‑AM classification (codes F00–F99) or is one of a number of other selected diagnoses (technical information).