Minister of Customs and electorate MP for Ikaroa-Rāwhiti Meka Whaitiri has stood down from her ministerial portfolios.
A statement by the Prime Minister says an investigation is being carried out "into a staffing matter in her ministerial office".
1 NEWS understands Ms Whaitiri is facing an allegation of assaulting a staff member within her ministerial office.
The Labour MP is also Associate Minister of Agriculture, Forestry, Local Government and Crown/Māori Relations.
Ms Whaitiri won the Ikaroa-Rāwhiti seat in a 2013 by-election, a seat held by the late Parekura Horomia. She is outside Cabinet.
She is a member of the Transport and Infrastructure Committee.
"Meka Whaitiri has told me she will be fully cooperating with the investigation, which will be thorough and conducted as quickly as possible," said Ms Ardern.
Ms Ardern said she was made aware of the "staffing matter" yesterday evening.
Labour MP Kris Faafoi will take up the role of Acting Minister of Customs, and her associate ministerial responsibilities will go back to the lead ministers of those roles.