Not sure what the dodgy employer is doing but in regards to non lodgement of tax returns, I reported my ex to the ATO as he hadn't done one since 2002-2003 financial year. Non lodgement is considered a form of tax evasion. Anyway, last year he was made to do his tax returns and I got paid a large majority of the money he had been owing me.

I did the reporting online and gave the ATO all the information I had in regards to him.

Not sure if it is an avenue you're wanting to go down but it's worth a try if like me, you'd tried everything else. Good luck