Simon Bridges says Jacinda Ardern should have dealt with Meka Whaitiri like "Key or Clark" would have.
"What I would have done is pretty simple - got the minister in and ask her what happened," he told The AM Show on Monday.
Ms Whaitiri has been relieved of her portfolios while an investigation into a staffing matter is carried out. Newshub understands there have been allegations of shoving.
"Meka Whaitiri has told me she will be fully cooperating with the investigation, which will be thorough and conducted as quickly as possible," Ms Ardern said on Thursday.
Mr Bridges says there shouldn't need to be an investigation if Ms Whaitiri pushed anyone.
"I can tell you what Key or Clark would have done - it's pretty simple... I would have thought it's a pretty simple conversation between Prime Minister and minister, and it should be able to be resolved pretty quickly."
He said while there are "always" disputes in ministers' offices, "pushing, shoving, physical contact is a different thing altogether".
"If Meka Whaitiri can't tell the Prime Minister what's happening, that's enough as well because a Prime Minister needs confidence in her."
Ms Whaitiri is Minister of Customs and Associate Minister for Agriculture, Forestry, Local Government and Crown/Māori Relations. While relieved of her portfolios for now, she remains co-chair of Labour's Māori caucus.
"In this country we go through certain processes, and she has to go through a process," fellow Labour Maori MP Willie Jackson told Newshub Nation on Saturday.
"She is still the MP for Ikaroa-Rāwhiti. She is still the co-chair, along with myself, for the Māori caucus. And I think all New Zealanders would agree there has got to be an investigation and a process to go through before we try to shut Meka Whaitiri down."
Serial litigant Graham McCready has filed a complaint with police, saying if they won't investigate, he'll take a private prosecution. He's successfully laid complaints against politicians John Banks and Trevor Mallard before, though Mr Banks was later exonerated.
Ministerial Services is carrying out the investigation.
Mr Bridges says he thinks Ms Ardern already knows what happened, but has been "weakened" by a string of blunders from her subordinates.
"She's had [Clare] Curran, she's had the Young Labour camp where no one's been held accountable. She's had Shane Jones in open defiance of her on her business charm offensive, then she's topped it off with Meka Whaitiri."
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