I guess in the past they could have had a job quoted at $500K. Paid "in cash" with $400K and reported only $200K to the tax office.
Those businesses could still just have $200K paid by electronic transfer (reported to ATO) and the rest in cash. It does mean the tax dodging responsibility is somewhat pushed to the other party too which might make some people/businesses uncomfortable and avoid it entirely.
I am all for more fairness and having people pay the right tax. Too many builders and tradies sort things out in cash. The unfairness then extends further where people then become eligible for government subsidies because on paper they earn less - but really have just reported less.
edit: reworded for clarity.