Former Prime Minister Sir John Key has warned the global economy is starting to splutter and that in the event of a downturn the parties in power in New Zealand are not the ones to set it right.

Sir John, who is now on the board of ANZ and Air New Zealand, made the comments at the National Party annual conference in Auckland.

National's finance spokeswoman Amy Adams released figures at the conference yesterday which showed New Zealand had the second lowest business confidence in the developed world.

"I think if you look at the economy on a general basis we are at the end of what I would say is the economic cycle," Sir John told reporters.

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"There's no question that when I look around in the world at the things I am now involved in internationally, you can start to see the pressure in the system.

"I was in China a week ago, it is clear that their economy is really starting to splutter a little bit along.

"While [in] the United States the economy is doing well, they are running massive, massive deficits, 7 per cent of GDP, [and] Europe's obviously much weaker than it was.

"So I think you are starting to see a slowdown in the economy. And I think that in part reflects the business confidence numbers in New Zealand.

"In part, New Zealand businesses are looking at what the Government's doing and are uncertain about that."

Sir John said he was hopeful that a downturn would be avoided because it would impact all New Zealand households.

"If it doesn't, then I think the right political party won't be the one that's currently there."

Key is attending the National Party annual conference for the first time since his surprise resignation in late 2016.

He kept a low profile after his departure from politics, in part to avoid overshadowing his successor Bill English and new National leader Simon Bridges.

Sir John said he was not at the conference at Bridges' request but because he would be a National Party supporter "until the day I die".

Asked what advice he gave Bridges, he said to "stay the course".

"National has some core values and principles that underpin this party and have done so for over 80 years.

"But also he has to put his own stamp on the party and shouldn't be afraid of having new ideas.

"Every administration has to change and reflect a modern New Zealand"

However, he would not pinpoint what areas Bridges might change.

He rejected any suggestion Bridges was not resonating with the public, saying it was a more difficult task being opposition leader against a new, first-term Government.

Soon after Sir John became National leader in 2006 his approval rating was around 24 per cent, compared to Bridges at 9 per cent.

Sir John said he had an advantage over Bridges because he became leader when "the tide was going out" on the Helen Clark-led Labour Government.