Tax and union corruption are set to dominate the return of parliament for the year.
The government will seek crossbench support on Tuesday for laws to bring back the Australian Building and Construction Commission, which it says is needed to weed out corruption in the industry.
However, Labor and the Greens say it unfairly singles out workers in one industry and denies them the same legal rights as exist in the court system.
"Why is it that if you've got a blue collar this government throws a royal commission and draconian legislation at you but if you have a white collar they turn a blind eye?" Greens spokesman Adam Bandt told reporters in Canberra.
Employment Minister Michaelia Cash has offered to show some crossbenchers confidential sections of the unions royal commission report in order to win them over.
Labor wants to steer attention away from unions and on to the potential for the GST to rise to 15 per cent under a re-elected coalition government.
"We're on the right side of history because we understand how real people are battling to make ends meet," Opposition Leader Bill Shorten told reporters in Melbourne.
Treasurer Scott Morrison said the government's tax package would be known before the election, due in September, and any voter disenchantment with the changes could be turned around.
"I campaigned heavily on what were very unpopular measures, whether it was on (boat) turnbacks or other things," Mr Morrison said.
"We did it in the way we said we were going to do it and we got the result we said we were going to get."
Greens leader Richard Di Natale said the major parties were ignoring higher taxes on the big end of town because they were benefiting from corporate political donations.
"I just think you've got to join the dots," he said of new figures showing mining companies and property developers were among those donating $179 million to political parties in the past financial year.
There was potential for tens of billions of dollars in extra revenue from ditching high-end superannuation tax breaks, negative gearing and capital gains tax concessions and the diesel fuel rebate, Senator Di Natale said.
Standing up for mining companies over farmers in the New England region of NSW could bring Nationals leadership aspirant Barnaby Joyce unstuck, he said.
Mr Joyce is widely tipped to take over the Nationals leadership if Warren Truss signals his retirement in the coming sitting fortnight.
The Nationals deputy leader told reporters the party had a depth of talent and any vacancies would attract a large field of candidates.
Labor frontbencher Tony Burke said the Nationals' instability would feed into broader unrest among conservatives in the coalition about Mr Turnbull taking the government down a moderate path.
"I've come in here today to Canberra ready for a big political fight with Malcolm Turnbull - unfortunately about half of his own party is coming here for the same reason," Mr Burke said.
