Prime Minister Frank Bainimarama said Australia should recognise the threat climate change poses to Pacific island nations.
"I appeal to Australia to do everything possible to achieve a rapid transition from coal to energy sources that do not contribute to climate change," he told a function in Tuvalu, which will this week hold the annual Pacific Islands Forum.
Some low-lying Pacific island nations are threatened by rising seas, while others are pummelled by cyclones that have become more regular and powerful due to climate change.
There has been disquiet in the Pacific that Australia -- led by climate-sceptic Prime Minister Scott Morrison -- recently approved the giant Adani coal mine in Queensland state.
"We face an existential threat that you don't face and challenges we expect your governments and people to more fully appreciate," Bainimarama said.
He added: "Put simply, the case for coal as an energy source cannot continue to be made if every nation is to meet the net zero emission target by 2050."
Smaller members of the 18-nation Pacific Islands Forum have been unusually vocal in criticising Australia's climate policies ahead of this year's summit amid a diplomatic push from Canberra to counter China's growing influence in the region.
High-level representatives from the likes of Tuvalu, Palau and Vanuatu have criticised their powerful neighbour for not doing enough to halt global warming.
Bainimarama, who seized power in a 2006 military coup but has since reinvented himself as a climate campaigner, is making his first appearance at the forum in a decade.
He has been sharply critical of Australia in the past and is unlikely to let Morrison's concerns, which centre on Beijing's activities, to dominate the Pacific Islands Forum agenda at the expense of climate change.
Oxfam Australia's climate change adviser Simon Bradshaw said the summit in Tuvalu was shaping as a key test for Canberra.
"If Australia is to remain a trusted partner to the members of the Pacific family, and with that retain the ability to help shape the region's future, it must immediately step up its response to the number one priority of its neighbours -– climate change," Bradshaw said.
The Pacific Islands Forum summit will officially open on Tuesday and continue until Thursday.