If the discrimination has led to the termination of your employment you must start any claim in the Fair Work Commission within 21 days from termination.
If you want to make a complaint about behaviour through the Australian Human Rights Commission, you have 6 months from the date the behaviour occurred. If your claim will go through the ACT Human Rights Commission, you have longer — up to 2 years. And if you want to complain to the Fair Work Commission about discrimination that did not result in a termination, you have up to 6 years.
Because the choice of jurisdiction depends on quite a number of factors, it is important that you seek advice as quickly as possible — particularly if you have been terminated.