With a political career spanning more than a decade, former conservative New Zealand prime minister John Key has shaken his fair share of hands along the way — including those of five Australian prime ministers.

"I used to say, 'mate, I don't really mind who turns up, just wear a name badge so I know who it is'," he quipped.

While divisions within the Liberal-National Party continue to grip Federal Parliament, Mr Key took aim at Australia's revolving-door politics during a jaunt to the Northern Territory, labelling it similar to instability in Italy.

The former PM enjoyed a close personal relationship with Malcolm Turnbull — with the latter even texting "say it ain't so, bro" upon news of Mr Key's retirement in 2016.

"I wasn't as sophisticated as that," he laughed.

"But I did ring him obviously about what happened ... I'm good friends with Malcolm, so you always feel when a political leader loses their job, that personal element, if you like.

"But also, in the time that he was PM, I thought he did a good job for Australia."

Mr Key said he was not surprised divisions within the coalition had resulted in another leadership coup, conceding it was part of the "rough and tumble" of politics, irrespective of party.

But he said Australia had endured a long period of political volitility.

"I don't want to say it, but you're looking a bit like Italy at the moment," he said.

"I was the 38th prime minister of New Zealand and John Howard was 24 — you're now up to 30."

Good public policy 'shouldn't be squandered'

Mr Key was giving a keynote speech at the annual Property Congress conference in Darwin, focusing on his experience driving reform during "challenging times".

When asked if political instability could jeopardise Australia's reputation, Mr Key said leaders could not rest on their laurels.

Mr Key met five Australian prime ministers during his time in leadership. ( AAP: Alan Porritt )

"All political leaders, it doesn't matter what party they come from, good public policy really makes a difference, and it can have a huge impact," he said.

"Australia's had 28 years of unbroken growth and that's really underpinned by a strong natural resource centre

"The challenge is to make sure you overlay that with good public policy and don't squander it, because like everything in life, it can go away quickly."

The tongue-in-cheek commentary from New Zealand wasn't restricted to Australia's political woes.

Mr Key recounted receiving a phone call from former British prime minister David Cameron during a fishing trip — just as a wayward tuna broke loose on the boat.

"He said 'what are you doing?' I said, 'I'm in the Marshall Islands, I just caught a tuna and my police officer is beating it to death as we speak'," he said.

"And there was this long pause and he goes, 'jeez, I wish I ran a small country'.

"Although, some could kind of argue he almost achieved that objective."