If you do high risk construction work then yes, you need a safe work method statement by law. Each state has different legislation in regard to what is considered high risk construction work which can make things confusing for builders and trade contractors who want to do the right thing. Here is some more information on what a SWMS is.
And not only is it difficult to find the information sometimes, it is then difficult to try and decide whether it applies to you. But generally, if you are working on a construction site, you are going to be doing some kind of high risk construction work, so it is best to have one, just in case.
Remember there is no monetary value placed on when a SWMS is required. Many clients believe that if a job is under a certain amount that it is not needed, this is false. Even if you are doing a job that costs $0 but you are doing high risk construction work, then under most states legislation you should have a SWMS for that work.