Malcolm Turnbull says President Donald Trump deserves credit for having a "red hot go" at denuclearising the Korean peninsula.
But the prime minister sounded a note of warning about Kim Jong-un's level of commitment to getting rid of its nuclear weapons.
North Korea has regularly signed denuclearisation agreements since 1992, only to continue to develop nuclear weapons in secret.
"There have been many disappointments along the road to denuclearisation of the Korean Peninsula. It clearly hasn't occurred," Mr Turnbull told reporters in Canberra.
"So he's giving it a red-hot go. We welcome it, but we do so clear-eyed and with caution."
Foreign Minister Julie Bishop said the focus was now on North Korea's response.
"The test now is this declaration that was signed with the US President in front of the world's media that must stand, and the onus now is on North Korea to prove that it's genuine," she told reporters.
That four-point agreement had scant detail about how to verify any steps North Korea takes toward complete and irreversible denuclearisation.
It also did not mention points raised later by Mr Trump around the abduction of Japanese citizens, human rights abuses or the withdrawal of US troops.
Ms Bishop doesn't believe the non-inclusion of those points means Mr Kim isn't open to further discussions.
"It was one meeting and we should never have expected that everything would be resolved in one meeting," she said, adding it could take months or years of further work.
Opposition Leader Bill Shorten agreed the summit was a positive step forward, "even if some of the images were certainly unusual".
"I wouldn't be opening the champagne bottles and popping the corks. People have been down this road before, but I'm not also going to be unduly negative," Mr Shorten told reporters in Perth.
Denuclearisation will be the first step toward peace on the peninsula and Australia and the International Atomic Energy Agency have both offered to verify the progress.
Mr Trump and Mr Kim promised to work towards building "a lasting and stable peace" on the Korean peninsula and to repatriate remains of prisoners of war and those missing in action during the Korean War.
Ms Bishop hopes this includes the remains of 43 Australians listed as missing in action in North Korea.
Tuesday's summit was the first time a sitting US president had met face-to-face with a North Korean leader.
Mr Trump said he had formed a "special bond" with Mr Kim, who said he and Mr Trump had "decided to leave the past behind".