Those considering running for National's leadership are expected to start showing their hands from today and are already canvassing colleagues before making up their minds.

Those believed interested include Amy Adams, Simon Bridges, Mark Mitchell, Jonathan Coleman, and Judith Collins.

There is also speculation Steven Joyce might stand although he is unlikely to publicly put his name up unless he knows he has support.

English's deputy Paula Bennett is yet to say what her own plans are but may be considering it herself.

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Some of the contenders could run on a ticket with a deputy.

The pack is likely to narrow down to two or three by the time caucus votes on it. That could be as soon as next week rather than letting it drag out, although English's resignation takes effect on February 27.

Those weighing it up were already talking to some colleagues to assess support last night, but none would publicly comment yesterday, saying it was English's day.

Despite recent talk among MPs about the successors to English and his deputy Paula Bennett, English's resignation caught many by surprise.

Collins, who is in the right faction of the party, was picking up some tongue-in-cheek support on social media – most notably from Green Party MP Julie Anne Genter. Collins has endorsed Genter as Green co-leader. In response to the news, Genter tweeted she was backing Collins: It's time for a woman who cares about transport. You're the one, @JudithCollinsMP! #gocrusher

Who are they:

Amy Adams (46): Selwyn MP. Former lawyer and Justice Minister, raised by a solo mum. Credited for National's work on domestic violence. Supported gay marriage and delivered pardon for those convicted for homosexual acts in the past.

Paula Bennett (48): Bill English's Deputy since last year. Her background as a solo mother stood out in National. Oversaw reform of welfare system. High profile and often underestimated.

Simon Bridges (40): Raised in Auckland, a former lawyer and Crown prosecutor who beat NZ First leader Winston Peters in the Tauranga seat in 2008. Former economic development and transport minister. Loves roads of national significance.

Judith Collins (58): Papakura MP since 2005. Former lawyer dubbed "Crusher" for war on boy racer cars. Fearless. Stood down briefly by John Key after "Dirty Politics" appeared to link her to efforts to undermine SFO, but inquiry found no evidence. Withdrew from previous leadership election, won by Bill English.

Mark Mitchell (49): The popular former cop and security contractor is MP for Rodney and former Defence Minister. Married to Peggy Bourne, widow of rally driver Possum Bourne. Likes dogs.

Nikki Kaye (38): Auckland Central MP and former minister. Beat PM Jacinda Ardern twice in Auckland Central. Treated for breast cancer – and a regular at the Coast to Coast. Supports gay rights and environmental protection. Has some public support – but unlikely to stand. This time.

Jonathan Coleman (51): Northcote MP. Former doctor – including for the Flying Doctor Service in Australia. Was Defence and Health Minister in government. Briefly contested leadership when John Key stepped down, but withdrew when English had the numbers.

Steven Joyce (54): List MP and former DJ with an inglorious academic record. Founded the RadioWorks empire. National's campaign maestro since 2005. Entered Parliament in 2008, appointed to Cabinet straight away. May stand but may also follow English out the door.