My understanding, based on discussions with structural engineers and consultants in relation to compliance with the Building Code is as follows.
You are not under an obligation to actively bring a building, or part of a building up to code. For example, you may have a building that was built in 1930 or 1970 in accordance with the Building Code that applied at the time. The Building Code changes all the time, you can’t expect buildings to comply with every single aspect of something that keeps changing.
However if you do work on a building, or make improvements or alterations to it, then anything related to that work has to comply with the Building Code, to the extent that that is possible. If something reaches the end of its life and has to be replaced then it has to be replaced with something that complies with the Code.
In your case the fact that someone can smell a lot of cooking coming from another unit doesn’t mean that the ventilation is not up to code. It may mean that the occupant of the unit cooks very smelly things, or that they don’t turn the fan on, or that the person complaining is very sensitive to smell. It may also mean that the fan is not working as well as it should.
The BCA standard for kitchen ventilation is:
a fan capable of a minimum extraction capacity of 50 litres per second when installed
(emphasis added)
The fan may have that extraction capacity, perhaps it could be checked. One thing I would ask the complainant is how they are so sure that the fan is not up to code, because the fact that they can smell something does not mean that the fan doesn’t meet the standard. I certainly don’t think that it is up to the OC to investigate every single thing like that, perhaps a good start would be talking to the occupants of the unit about what they are cooking and whether they are actually using the fan?
The residual question is whether the fan is the responsibility of the OC. In our scheme the owners have all changed their fans over time and they are responsible for them.