A former National MP says one or two leadership candidates could drop out this morning before the vote.

No one's conceded yet, but Craig Foss says most MPs would know who's likely to get past the first round of voting.

The lowest polling candidate will drop out each round, until one candidate has 29 votes in a caucus of 56.

It's a secret ballot so no one will know who votes for who.

Foss told Kate Hawkesby most will have decided who they want in the top job before this morning's final five-minute pitches.

"Some people might have told more than one person they'll vote for them, they may have said they're undecided, they may have taken soundings from the electorate last night or over the last few days. But they will go in knowing who they're voting for."

Foss says whatever the outcome and feelings about the new leader, the party just has to get on with it and display a united front.

"There can be a pretty strong debate behind closed doors, but once that door is opened, everyone lines up behind the new leader whoever it is."

LISTEN TO CRAIG FOSS TALK WITH KATE HAWKESBY ABOVE