Opposition parties will target Prime Minister John Key in parliament on Tuesday amid mounting speculation that the Conservative Party will replace ACT as the government's ally.

Mr Key is in the firing line because ACT's leader and sole MP John Banks is surrounded by a donations scandal and his party is expected to suffer the consequences at the next election in 2014.

ACT has a support agreement with the government and Mr Banks holds ministerial portfolios.

Labour, the Greens and NZ First say he should be stood down but Mr Key is refusing to do that.

He says Mr Banks has given him an assurance he didn't breach electoral law when he listed two $25,000 donations from internet tycoon and piracy accused Kim Dotcom as anonymous, although Dotcom says he knew where they came from and called to say thanks for the money.

Police are investigating the donations, as well as $15,000 from Sky City which Mr Banks also listed as anonymous.

Mr Banks was given the donations in 2010 when he made a bid for the Auckland mayoralty.

The Conservative Party won 2.6 per cent of the party vote in last year's election but didn't get any MPs because its leader, Colin Craig, failed to win an electorate seat.

ACT won only 1.07 per cent but Mr Banks - with National's help - won the Epsom seat.

Mr Craig says he's happy to work with National and Mr Key is leaving the door open.

"The government I lead has proven to be centre-right, moderate, pragmatic and economically and fiscally conservative," he said on Monday.

"It seems to me that Colin Craig and the Conservative Party fall broadly into that category."