National MP Nikki Kaye has taken over as acting National and Opposition Leader, after Todd Muller's resignation.
Ms Kaye was Mr Muller's deputy.
National MPs will meet this evening after rushing to the capital following Mr Muller's announcement this morning.
"It has become clear to me that I am not the best person to be leader of the Opposition and leader of the New Zealand National Party at this critical time for New Zealand," Mr Muller said.
Mr Muller took over as National Party leader on May 22, when he rolled then-leader Simon Bridges. Ms Kaye took over as deputy from Paula Bennett - who announced earlier this month her decision not to run again at the September election.
Ms Kaye is the MP for Auckland Central.
Prime Minister Jacinda Ardern says she passed her best wishes to Todd Muller and his family, after the National leader announced his resignation in a surprise statement this morning.
"No matter what side of Parliament you're sitting, politics is a difficult place."
Deputy Prime Minister Winston Peters took some shots this morning at the National Party, saying, "National has demonstrated to voters as clearly as it is able that it cannot govern itself".