OPINION: The National Party could have a new leader as soon as Tuesday night, but probably not according to one MP.
This is uncharted territory for the party and no process is yet in place to determine what happens next.
All MPs know is that they're rocking up to Wellington for the 7pm caucus and then begins the process to find the next National Party leader.
Watch the Newshub Special on the National Party leadership crisis live tonight from 7.30pm on Newshub.co.nz.
Campaign hoardings with Todd Muller's mug have been sent out to the electorates. MPs have brought family, friends, volunteers, every willing and able being in to help put them together ready to be rolled out around the country. It's hard yakka and expensive work.
But no one saw this coming. Not least Muller himself.
Newshub understands most MPs only found out about Muller's resignation when the press statement was sent out at 7:30am on Tuesday morning, and he was unable to make it to an emergency teleconference caucus call.
Muller's statement said the leadership role had "become untenable from a health perspective".
Newshub understands the statement relates to his mental health.
Tova O'Brien is Newshub's political editor.