Labour Party leader David Cunliffe says it is difficult to contemplate someone who operates outside the law serving as a minister in a government.
He was responding to comments by Mana Party leader Hone Harawira at the party's AGM in Rotorua that Mana should be viewed as a movement which is often going to step outside the law.
Mr Cunliffe said the remarks were unwise. "Nobody and no member of Parliament should be operating outside the law - I wouldn't condone that for a moment," he said.
"(Mr Harawira) will have to be very, very careful making statements like that because it will invite the kind of close scrutiny that he probably doesn't want."
Mr Cunliffe said it would make him think twice about entering into any political deals with Mr Harawira.
Political scientist Bryce Edwards said Mr Harawira is making such comments to try to offset his support of Kim Dotcom's Internet party and to reassert his revolutionary credentials.
Mr Harawira has backed the idea of aligning with the Internet Party, saying he thinks it could increase Mana's numbers in Parliament.
"Hone Harawira is vulnerable to being seen as being an opportunist and ... selling out his more radical roots," said Dr Edwards.
"That's why we're seeing some more radical statements from Hone Harawira this weekend - just to show his activists and supporters that he's not selling out by joining up with Kim Dotcom, that he hasn't gone moderate."
Mr Dotcom on Saturday urged about 200 delegates at Mana's annual meeting to form an alliance with his new party in a bid to bring down the National-led Government. After late-night deliberations, party members voted within eight different rohe (tribal territories) and all groups decided to continue talking with the internet businessman.