South Australia was an irregular attender at first. She did not send delegates to the Federal Council (once IIRC). However, she did send representatives to the First National Australasian Convention and adopted reforms to her Constitution in 1891, so you would need a POD prior to that.
So....
You might want to eliminate Sir John Downer, Thomas Playford II, Charles Kingston or Sir Frederick Holder, or find some way for one or more of these to be affected so as to become a virulent opponent of Federation. Or maybe have Playford decide that he is too busy being Governor of India to push the Federation cause. That would delay things.
Also maybe the smaller states don't reach agreement with New South Wales, and so Tasmania, South Australia and Victoria form their own federation in 1892. They remained the Commonwealth of South Australia until they decide to become a republic in the 1980's.