No, carbon monoxide detectors are not mandatory in all Australian homes, although they are recommended, especially in homes with gas appliances. While some public housing has a requirement for CO detectors, they are not as standard as smoke alarms.
Here is a more detailed explanation from anka security:

Not mandatory:

Unlike smoke alarms, most Australian homes do not require carbon monoxide detectors.

Recommended:

Although not mandatory, consumer groups and safety organizations strongly recommend the installation of carbon monoxide detectors, especially in homes with gas appliances, fireplaces or attached garages.

Social housing exceptions:

Some carbon monoxide detector requirements exist for social housing in certain areas.

Risk awareness:

A significant number of Australians are unaware of the dangers of carbon monoxide and the importance of carbon monoxide detectors.

Silent killer:

Carbon monoxide is a colorless, odorless, tasteless gas that is difficult to detect without a detector.

Potential dangers:

Gas heaters, stoves, fireplaces, and even cars in garages can produce carbon monoxide.

Prevention:

Installing carbon monoxide detectors is a key step in preventing carbon monoxide poisoning, which can be fatal.

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