Labour and the Greens aren't happy with proposed legislation to replace the repealed Electoral Finance Act.
The proposals, released on Tuesday by Justice Minister Simon Power, would allow lobby groups - known as parallel campaigners - to spend as much as they want during an election campaign although if it is more than $12,000 they would have to be identified on a register.
Donations that political parties receive would be disclosed in bands, the regulated campaign period would remain at three months and there would be no change to the broadcasting regime.
It is the parallel campaigning proposal that is worrying opposition parties.
The previous Labour government introduced the Electoral Finance Act which limited groups outside political parties to spending $120,000 and they had to be identified.
That was after the Exclusive Brethren ran an initially covert campaign against Labour and the Greens in the 2005 election.
National campaigned against the Electoral Finance Act and repealed it soon after they won the 2008 election.
Labour's justice spokeswoman, Lianne Dalziel, said the proposed legislation didn't limit the amount parallel campaigners could spend or require them to say how much they had spent, or how closely they were allowed to work with political parties.
"This still risks creating the sort of free-for-all that occurs in the United States where campaign spending is out of control," she said.
"Labour's Electoral Finance Act was a direct response to the subterfuge and political chicanery that characterised parallel campaigning in the 2005 election."
The Greens voiced similar concerns, but the ACT Party supported the proposals.
"We were always against restricting the freedoms of other New Zealanders and have said we will support this bill to the select committee and have a good look at it," said ACT leader Rodney Hide.