Government minister Nikki Kaye has been diagnosed with breast cancer and will be taking leave from her ministerial portfolios.
Ms Kaye, 36, says she received the "devastating" news on Friday.
"Having the opportunity to serve NZ as the MP for Auckland central and a cabinet minister continues to be a huge privilege. I have always worked hard and given everything I have to both roles," she wrote in a Facebook post.
Prime Minister John Key has re-assigned her portfolios.
"I have spoken with Nikki and assured her she has the full support of her colleagues and I as she deals with this difficult diagnosis," Mr Key said.
"Her medical team is working hard to ensure a full recovery. Nikki will be dedicating her energy towards getting well, and I wish her all the best."
Mr Key said Ms Kaye was "young, she's very fit and she's very determined and she's a fighter so I'm sure she's going to get through this and be back with us".
He said it was likely she'd be away for months.
In the meantime, Gerry Brownlee will act as Minister of Civil Defence, Nathan Guy will act as Minister for ACC, Anne Tolley will act as Minister for Youth and the Associate Education responsibilities will be overseen by Hekia Parata.
Ms Kaye says plans will be in place for her electorate office to make sure staff can still cater to constituents' "personal issues, policy ideas or queries".
"I am fortunate to have wonderful hardworking staff and National party colleagues who are helping ensure this support is there for my constituents.
"I feel so lucky to have such an amazing family and group of friends who are giving me buckets of love and have been so strong helping me work through this."
Ms Kaye has asked for "a bit of space and privacy" while she undergoes treatment.