Labour Party MP Pete Hodgson has accused Environment Minister Nick Smith of having a conflict of interest when he sacked the Canterbury Regional Council.
During Parliament's general debate on Wednesday, Mr Hodgson told MPs that in June last year the council and police visited the property of Dr Smith's brother, Tim Smith.
He says 21 charges were subsequently laid, to which Tim Smith pleaded guilty.
Mr Hodgson says the Cabinet manual makes it clear that Dr Smith had a conflict of interest.
He says Dr Smith should have transferred responsibility for the matter to another minister, but he did not.
A spokesperson for Dr Smith says the minister was unaware of any prosecution involving any sibling until after the decisions were made by the Cabinet on the regional council.
He says Dr Smith rejects any assertion that he breached the Cabinet manual and is disappointed Labour dragged family members into political debate.