Gillard the liar, page-50

"Thiess didn't pursue it because they were afraid of the unions"

It goes further than what you posit.
Unions and big contractors can have a symbiotic relationship.
This relationship can ensure the big players like Lend Lease and Leighton keep the competition out of the market so they can charge more.
IIRC with respect to WA a Thiess employee was related to someone in the union so don't know why they would be afraid of the union.

Also i recall that some payments may be related to the union turning a blind eye to using contaminated fill.