National MP for Rangitata Andrew Falloon has announced he will be retiring at the election.

Announcing the move in a statement after 2pm, the first-term MP for Rangitata, in Canterbury, said he was receiving counselling after the death of a friend.

He said he had lost three close friends to suicide when he was younger, and another friend taking their own life had brought back "much unresolved grief".

"Recent events have compounded that situation and reminded me of the need to maintain my own health and wellbeing. I have again been receiving counselling," he said.

"I have made a number of mistakes and I apologise to those who have been affected."

He thanked National Party leader Judith Collins for her support, and said he looked forward to helping a new candidate for the electorate.

"I apologise for this disruption to my colleagues and to those I serve in Mid and South Canterbury."

Collins also released a statement, saying the party was advised late of "an issue relating to Andrew late on Friday afternoon", and had dealt with it this morning.

"Andrew is suffering from significant mental health issues and his privacy, and that of his family, must be respected," Collins said.

The departure comes after a wild fortnight for the opposition, with a sudden change in leader and several high-profile resignations.

Main Image: National MP for Rangitata Andrew Falloon with Gerry Brownlee and then-leader Todd Muller announcing the party's policy for a four-lane highway between Christchurch and Ashburton. Photo: RNZ / Nate McKinnon