You have to have a permanent visa that states 'indefinitely' on it (this is isngapore airlines) to get your 40kg. However you only get this is it is the first time that you travel on the visa, so if you have already done a validation visit you will only get the standard 20kg.
Hope this helps
Singapore Ailines Baggage Allowance
Originally Posted by Devon
I've just spoken to Singapore airlines, and if your visa says on it - 'Permitted to stay in Australia indefinitely' - you will get your allowance doubled to 40kgs. Not the case with Quantas, they only allow 20kgs.
Or if you fly via the USA, you get 40kgs anyway.