Todd Muller wasn't on the emergency caucus call this morning when the National Party learned of his resignation as leader, 1 NEWS understands.
At around 7.30am today, Mr Muller released a sudden statement announcing he'd be retiring as leader of the Opposition and the National Party effective immediately.
At the same time, his party's MPs learned the same news, 1 NEWS understands.
The caucus met on an emergency teleconference call, where deputy leader Nikki Kaye read out a statement.
Several MPs say they were surprised and shocked by the news.
Now National MPs are scrambling to get flights back to Wellington ahead of a 7pm caucus meeting at Parliament, where they'll vote on a new leader.
In his statement this morning, Mr Muller said the role had taken a "heavy toll on me personally, and on my family".
"This has become untenable from a health perspective," he says.
It comes after a challenging week for the party, after private Covid-19 patient data was leaked by a National MP.
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