What the law states:  
(hia.com.au/bathroom ventilation/30 July 2000)
Volume 2 of the BCA covers ventilation requirements for housing in Part 3.8.5.  The Acceptable Construction Practice is specifically covered in Clause 3.8.5.2 (a), (b) and (c).

•    Firstly, if a window is installed in the room, then it becomes the primary source of ventilation for the room. Therefore, irrespective of whether an exhaust fan is installed, if the room in question has a window installed in accordance with the provisions of sub-clause (a) then the ventilation requirements for the room complies with the BCA.
•    Similarly if the room has ventilation borrowed from an adjoining room in accordance with sub-clause (b) it also complies with the BCA.
•    If installed, an exhaust fan only needs to comply with sub-clause (c) if it is the only source of ventilation to the room.
Therefore in accordance with sub-clause (c),
•    open eaves;
•    roof vents, including a vented ridge;
•    a tiled roof without sarking

Fan Extraction: (manrose/ventilation/introduction/fan regulations)

Bathrooms: The regulations require a fan capable of a minimum extraction capacity of 25 litres per second when installed.

Toilets: The regulations require a fan capable of a minimum extraction capacity of 25 litres per second when installed.

Kitchens: The regulations require a fan capable of a minimum extraction capacity of 50 litres per second when installed.

Laundries: The regulations require a fan capable of a minimum extraction capacity of 20 litres per second when installed.

Ventilation Requirements: (build.com.au/bca ventilation requirements)
When planning a ventilation system, there are a few building code requirements that you need to be aware of. In particular, the Building Code of Australia (BCA) states that you must have windows that are equal in size to at least 10% of the floor space, for the purpose of natural light – or 3%, for roof lights (or a proportional combination of windows and roof lights).

The 5% Ventilation Requirement:
For the purpose of ventilation, an openable window or similar aperture is required to be at least 5% of the floor space in that particular area. The area itself may be defined just as one particular room, or a combined set of adjoining spaces. Also, in a room where there is no ceiling fan, the BCA states that you must achieve effective cross ventilation.