National MP Nikki Kaye has returned to Parliament for the first time since she was diagnosed with breast cancer to participate in her party's leadership contest.

"I'm back, briefly," she told reporters. "I want to be here to participate in the process.

"I'm still getting some treatment but my plan is to be back early next year to resume full duties."

Kaye said she would back Deputy Prime Minister Bill English to take over from John Key, who announced his resignation yesterday.

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Following a National Party caucus meeting this morning, English and Health Minister Jonathan Coleman said they would be putting up their hands for the Prime Minister's job.

"I understand that the Deputy Prime Minister has put his name in the hat and I will be supporting him," Kaye said.

She was "gutted" by Key's resignation but said she was "delighted that Bronagh and Stephie and Max are going to have their back".

Kaye also confirmed she would stand again in Auckland Central in next year's election. She has held the seat since 2008 and reclaimed it in 2014 with the smallest winning margin in the country - 600 votes.

The 36 year-old National MP stood down from her ministerial portfolios in September to get treatment for breast cancer.

She spoke today about the support Prime Minister John Key had given her during her leave from Parliament.

Holding back tears, she said Key had given her "every bit of support".

"I don't want to cry, but he has been an incredible rock during this period.

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"So has the Deputy Prime Minister and the caucus.

"To all of those people who get cancer, and particularly breast cancer sufferers out there, I just want to acknowledge the courage that they have and to say that I could not have more support from my caucus during that period."

Kaye also thanked media for giving her the space to recover.