when are doctors required to compulsorily report patients in Australia

reportable deaths

http://www.health.gov.au/internet/main/publishing.nsf/Content/ageing-quality-guidelines-cr-ap.htm
compulsory reporting and protection requirements commenced on 1 July 2007 following amendments to The Aged Care Act 1997 (the Act)
report to the police and to the Department of Social Services within 24hrs incidents involving alleged or suspected reportable assaults:
- unlawful sexual contact with a resident of an aged care home, or
- unreasonable use of force on a resident of an aged care home

you are concerned about a child because you have:
- received a disclosure from a child about abuse or neglect
- observed indicators of abuse or neglect
- been made aware of possible harm via your involvement in the community external to your professional role
see Victorian child protection reporting flowchart (pdf)

see http://www.health.vic.gov.au/dpcs/reqhealth.htm
also Aust. Prescriber
- “Once drug-seeking behaviour has been identified or there is reasonable suspicion, the possible response is determined in part by the laws applying in the State or Territory. While these vary, in general it is illegal to prescribe solely to maintain someone's dependence. The only exception is those with special permission such as methadone prescribers. A second response is to contact State or Territory health authorities”

when you believe a patient is at risk to themself or others such that they need urgent assessment, there is a duty of care to contact police to assist in performing a welfare check or potentially finding the patient and bringing them back to hospital for assessment.

S79 of the NSW Firearms Act 1996 provides for the notification to the NSW Police Commissioner by certain health professionals if they are of the opinion that a person to whom they have been providing professional services may pose a threat to their own or public safety if in possession of a firearm.
most hospitals also have a Code Black for security concerns of persons with weapons - staff must contact police for assistance
most hospitals have a procedure for notifying police when a person gives staff a weapon:
- eg. “When surrendered the Security Shift in Charge must secure the item immediately in a lockable cabinet within the Security Office and contact the Police to advise that they are in possession of a firearm, prohibited weapon, controlled weapon or dangerous article.”

although reporting is not mandatory in many regions, doctors must still weigh their duty of care to the community with their duty of patient confidentiality
many clinicians would report cases where they believe a serious crime has occurred (eg. a murder, stabbing, shooting), or if the community is substantially at risk, such reporting preferably should be done with the patient's consent

in Victoria, Road Safety Act 1986, Marine Act 1988, and Rail Act allow for hospital staff to take blood samples from trauma cases which are then sent to Police via a strict procedure, patient refusal requires a refusal form to be completed and sent through the same mechanism and such refusal will generally result in loss of licence.

in Victoria there is NO mandatory reporting requirement for healthcare professionals HOWEVER you have an ethical obligation to report to VicRoads to support public safety if there is a belief the patient lacks insight or judgment and is not heeding advice to cease driving and to follow their legal obligation to self-report to VicRoads of a condition that would affect their ability to drive safely such as seizures.
see https://www.vicroads.vic.gov.au/licences/health-and-driving/information-for-health-professionals