UPDATED 7.13am: A candidate to be top dog of the National Party is talking up the need for unity after the party makes its choice.

LISTEN ABOVE: Matthew Hooton speaks to Rachel Smalley about the leadership race

Caucus will vote on Monday to fill the void about to be created by John Key's departure.

Jonathan Coleman is taking on Bill English and Judith Collins for the leader's role

He told Mr English of his intention to run before he went public.

"The other thing I emphasised - it's very important that whoever comes through this, the whole caucus gets behind that person and supports them, and that's absolutely crucial in the transition to new leadership."

The man tipped as the frontrunner to fill John Key's shoes is unfazed he has a battle on his hands.

Deputy Prime Minister Bill English said he's wasn't particularly surprised Dr Coleman put his hand up for the job.

"I know Jonathan well, he manages the biggest, hardest public agency in government and that's the health system - he does that very competently."

English added that he had "always known he was ambitious, so he's welcome to have a go."

It's thought Bill English has already made promises to people about who gets what, if he becomes Prime Minister.

Political commentator Matthew Hooton told Rachel Smalley the hierarchy of an English-led government would be similar to the status-quo.

"The Bill English ticket has emerged, it's English as PM, Paula Bennett as Deputy Prime Minister, Steven Joyce as Finance Minister, Murray McCully as Foreign Minister and a senior role for Gerry Brownlee."

Corrections and Justice Minister Judith Collins stepped down from Cabinet just before the last election after she was accused of undermining the Serious Fraud Office director.

She was reinstated a year ago today, and wants to put her chequered past behind her, saying she's learnt her lesson.

" One of the things that you learn in life is you learn from things that go wrong, and I've learned an awful lot."

Collins said she wouldn't still be there, unless she thought she was adding value.