Is at hand a minimum size of fire door within austalia? ...
Answers: Fire Doors are designed to last an hour or more back a fire can burn through them. Doors come in abundant sizes and styles.
ThermaTru is one of many manufacturer of these doors. The smallest door the list is 26 inches far-reaching.
There are standardized building codes in the U.S
Australia have them as well. This site have more info: http://www.dhw.wa.gov.au/193_745.asp
Primary doors are typically 36" wide. They should be fire doors, and must be fire doors contained by commercial buildings. I would imagine that your local building inspector would know rotten the top of his head if a minor door has a minimum size.
I hope this help. Good luck with it.