I’m working with a towing business that wants an automatic invoicing system using cash or bank transfer however he doesn't want to include card details, reason is so he can underreport income (lie) to Australia tax. If I build it with intention to help him collect payments, am I helping him in sin?

My question would be: are you aiding him in his trickery? If yes, then no, you must refuse. You should only do it in a way that is legal and without cheating. However, if it is something separate (for example, if he is going to do that separately and you just happen to know about it), then you are not aiding him because you are just making a system for payment. If he then goes on his own to do something like this—or he may or may not do it—then this is not forbidden.