Prime Minister Jacinda Ardern says she's passed on her best wishes to Todd Muller and his family, after the National leader announced his resignation in a surprise statement this morning.
Mr Muller says he's stepping down as National leader and as leader of the Opposition, effective immediately.
"The role has taken a heavy toll on me personally, and on my family, and this has become untenable from a health perspective," he said in a statement.
Ms Ardern has acknowledged his resignation and says she's passed on her best wishes to the MP and his family.
"No matter what side of Parliament you're sitting, politics is a difficult place," the Labour leader told 1 NEWS in a statement.
Mr Muller's resignation comes after a difficult week for the party, rocked by leaks and a privacy scandal.