Prime Minister John Key seemed genuine when he spoke about wanting to spend more time with family in his resignation speech today, a body language expert says.

Key announced he was standing down as Prime Minister at his weekly press conference this afternoon.

He cited family reasons for leaving, saying the job had required great sacrifices "from those who are dearest to me".

His wife Bronagh had endured "many lonely nights" and his children Stephie and Max had been put under "extraordinary levels of intrusion".

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Suzanne Masefield, of the Body Language Company, told the Herald Key appeared to be speaking honestly about wanting to spend more time with family.

"There was a lot of emotion in his body language when he was speaking about his family and the impact being Prime Minister had had on them," she said.

"His voice wavered and his breathing was laboured. He was clearly trying to contain his emotions."

But he didn't appear to be 100 per cent convinced the National Party was in great shape, or that they would win the 2017 election.

"When he talks about the party being in great shape and how capable Cabinet is, he looks up and around a lot.

"People generally do that when they don't completely believe what they're saying and so look to see if others are believing them."

Overall, Key seemed anxious and as if he wanted to get through the speech quickly.

"He seemed really, really uncomfortable. He was smacking his lips a lot and looking down a lot more than normal, suggesting he hadn't learned the speech, and this is something that has happened quite quickly.

"He was trying very hard to protect his emotions. He was shallow-breathing when he was speaking about his achievements, as though he wanted to rush through them."

Masefield believed Key was being honest when he said he wasn't sure what he would do next.

"When he says he doesn't know what he'll do, he looks down, suggesting he isn't certain. I expect he has some idea but doesn't have anything definite sorted."

The National Party caucus will hold a meeting on December 12 to decide the new party leader and Prime Minister.