Former prime minister Sir John Key has reminded National it needs to find potential coalition partners for next year's election.

Photo: RNZ / Dan Cook

And he has suggested New Zealand First may not be one of them, saying its leader Winston Peters had shown his colours.

Sir John is among those gathered at the opposition party's annual conference at Christchurch Town Hall this weekend where party members are meeting to take stock, discuss policy and scratch out tactics for 2020.

Speaking to RNZ, Sir John acknowledged National faced a "challenge" in ensuring it had coalition partners, as the MMP electoral system made it difficult for any individual party to have absolute control.

But he said National had to "continue to work" on finding new friends.

"There are a lot of opportunities out there for either potential parties or for other partners to grow and strengthen.

"We know the parties that won't work with us. It's a matter of finding some that will."

Sir John said starting a new party could be difficult, but he believed voters would be more open toward "deals" or new parties being started given the outcome of the 2017 election.

"The rules of the game changed a little bit when New Zealanders realised that Winston Peters effectively allowed the second place party to become the government."

In 2011, Sir John famously ruled out working with New Zealand First after that election.

Asked whether Mr Bridges should do the same, Sir John said that was "a matter now for the leadership".

But he suggested Mr Peters would be an unlikely partner for National.

"We saw Winston's colours on election night in 2017. The reality was that the majority of New Zealanders - 44 percent of them - voted for a National-led government and they didn't get one."

Sir John endorsed Mr Bridges' opening speech which included the repeated slogan: "Our bottom line is you".

"Opposition is always a difficult place, but they've come to the conference with the right messages. Ultimately, personalising politics is a very important thing to do.

"Always a lot is made about the popularity or otherwise of the leader, but the reality is people vote very much on the big issues - the economy, law and order, health and education."