National MP Todd Muller has arrived at Parliament ahead of his leadership battle against Simon Bridges.
The National Party was called back for today's emergency meeting to decide who would be Opposition leader.
"Feeling very excited, a momentous day for the National Party," Mr Muller told journalists.
"I look forward to the conversation with the caucus."
Mr Muller's wife, Michelle Muller, also arrived. When asked for comment, she said she was "just here to support my husband".
Yesterday, the 1 NEWS Colmar Brunton Poll showed National at its lowest support level since 2003, sitting at 29 per cent.
The winner of today's behind-closed-doors vote is likely to be announced this afternoon.