Polygamy is illegal and in Australia, bigamy (getting married while still legally married) is criminalised. The Marriage Act 1961 (Cth) provides that any person who, while married, goes through a form or ceremony of marriage with any other person is guilty of an offence with a maximum penalty of five years imprisonment [s 94(1)].
Many people who are not Muslim have multiple marriages, divorce, and children from multiple marriages. These people may be entitled to benefits, but these benefits all rely on looking at income and circumstances.
It is up to each individual applicant to Centrelink to provide truthful information, and intentionally failing to do that can amount to Centrelink fraud. Crimes under the Criminal Code such as Obtaining financial advantage [s 135.2(1)], Dishonesty causing a loss [s 135.1(5)] , and Obtaining a financial advantage by deception [s 134.2(1)] have penalties that range from 12 months to 10 years maximum imprisonment and large fines may also apply.
Centrelink fraud is illegal and is not unique to any one group. There is no need to specifically target Muslims to seek out where people are committing fraud. Any suspected fraud should be reported: https://www.humanservices.gov.au/customer/contact-us/reporting-fraud
In any event, Sharia law explicitly mandates Muslims to respect the law of the land in which they reside, so Muslims engaging in conduct that is in breach of Australian laws are engaging in un-Islamic conduct.