Rodney MP Mark Mitchell has announced he will run for the National Party leadership.
Mr Mitchell confirmed at a media conference this afternoon that he was in the running.
He said he was running for leader because he "believed strongly in what National stands for".
"National's values - strong families, personal responsibility, fiscal responsibility, looking after our vulnerable and our environment - are my values. They guide my decisions and are the foundation of the policies I'd campaign for as leader.
"But as we see too often in politics, talk is cheap, and words and values aren't enough by themselves."
Watch John Campbell's full Checkpoint interview with Mark Mitchell here:
To win the next election, National needed a leader who could take action and make the most of National's talents, he said.
"This isn't about personal ambition or who can be the toughest or the nicest. This is a serious decision for our caucus about who we want to see as the next Prime Minister."
He said Prime Minister Jacinda Ardern's vision "lacked any substance".
"She has no clear plan for this country and her government is making it up as it goes along. This simply isn't good enough."
The caucus will vote for the new leader next Tuesday.
Mr Mitchell said he was not interested in the deputy position if he did not win, and would not be standing unless he believed he had strong caucus support.
He said he was approached by some fellow MPs about three weeks ago about running for leader.
That was before Bill English resigned, but Mr Mitchell said he was not planning to mount a campaign against him.
"So I actually started to go through that process but of course we were all caught out of left field when Bill [English] announced...people were starting to obviously think about the future and so there had been some discussion around that."
However his intention was always to support Mr English as leader, said Mr Mitchell, and encourage him to stay on in that position.
Mr Mitchell is a former police officer and ran a private security company in the Middle East.
He entered Parliament in 2011 and was appointed Defence Minister in the previous National government.