By definition you cannot have a polygamous marriage in Australia without being married to two people at once, which would make you guilty of the criminal offence of bigamy. Thus, polygamy in Australia is not legal.

How do I report bigamy in Australia?

The local police station is the place to make the report, but you need to provide them with evidence. A copy of his marriage certificate and evidence that she has remarried, eg a newspaper notice or similar, but only after you have confirmed that she has not been granted a divorce.