Greens ask if treasurer can exempt homeowners in his electorate from CGT changes

Nick McKim (right) has raised questions about powers in the bill. (ABC News: Callum Flinn)

The Greens have indicated the party is willing to help Labor pass its tax laws

But they hold concerns about the specifics contained within the bill which could allow treasurers to "fundamentally alter" the laws

Meanwhile the opposition says it will work with anyone who could block the laws proposed by Labor

The Greens have seized upon powers within the government's signature tax reform bill, questioning whether it allows the treasurer to exempt home owners in his Queensland electorate.

While the party has indicated it is willing to help Labor pass the tax laws — which includes changes to negative gearing, the capital gains tax (CGT) discount, trusts and the $250 tax offset — by early July, the Greens hold concerns about the specifics contained within the bill.

The bill, which passed the lower house on Thursday unchanged, allows some asset definitions to be determined by the treasurer via legislative instrument, which can be overturned by the Senate.

Greens senator Nick McKim says the bill could potentially allow future treasurers to "fundamentally alter" the laws once they are passed in their current state.

Senator McKim questioned whether Treasurer Jim Chalmers would theoretically have the powers to exempt property owners in his home city of Logan from changes to CGT calculations.

"The minister could for example decide to exempt all houses in Logan from the new arrangements and allow those owners to have access to the very generous 50 per cent discount. Could the minister not do that?" Senator McKim asked in Senate Estimates on Thursday.

Katie Gallagher says future changes could be stopped in the Senate. (ABC News: Adam Kennedy)

He also asked if the government was worried about "future treasurers who might be from the Liberal Party or, horrifically, a One Nation treasurer, might use these powers to walk back some or all of your capital gains tax reforms".

Finance Minister Katie Gallagher noted the powers were "disallowable instruments", which mean any change a treasurer might make could be stopped by the Senate, which Labor does not control.

"The Senate has not been shy about disallowing things that it doesn't like," Senator Gallagher said.

Support from the Greens is crucial for the passage of the bill through the Senate.

Mr Chalmers said he was well aware of the concerns from the Greens and was in talks with the party, saying the powers contained within the laws were standard procedure.

"Nobody has the numbers in the Senate on their own, and so we