The Labour Party leader is questioning whether problems within Cabinet are behind fourth-ranked minister Simon Power's decision to quit politics.
The MP for Rangitikei, who holds the justice, commerce and state-owned enterprises portfolios, says he wants to explore opportunities in the private sector.
Prime Minister John Key said he was stunned and flabbergasted when Mr Power told him he would stand down at the election. He said Mr Power had the potential to be leader of the National Party.
Labour leader Phil Goff says it's very unusual for a first-time minister with a high ranking to leave politics. He expressed doubts about the simple explanation being offered for Mr Power's announcement.
But Mr Power said through a spokesman that suggestions of a split in Cabinet are laughable. He said there had been no ructions in caucus or Cabinet with Mr Power and his colleagues were shocked about his decision.