Scott Morrison has savaged a draft recommendation from UNESCO to list the Great Barrier Reef as "in danger".
The federal government suspects China has used its political clout on advisory bodies to engineer the outcome.
Various delegates to UNESCO from around the world have now co-signed an open letter denouncing the reef recommendation.
"The UNESCO process has been appalling," the prime minister told Brisbane radio 4BC on Thursday.
"We've been busy in talking to our friends and the list of countries is quite extraordinary."
UNESCO insists its recommendation is based on science, not political influence.
But Mr Morrison is furious about the apparent lack of transparency, due process and consultation.
"This process is not on. There's a proper way to do these things," he said.
"We invest, together with the Queensland government, some $3 billion on reef sciences, the best managed reef in the world.
"Sure, it's got challenges like sensitive environments all around the world, but Australia does it better than anywhere else, so we will be making that case."
The prime minister believes there has been a significant shift in UNESCO's approach.
"We really do think this process has been absolutely appalling, quite different to when this issue was dealt with by UNESCO early on in our government back in 2014," Mr Morrison said.
"We worked through that process and got the right outcome, but this process is a bit of a try-on."
The letter from ambassadors raised collective concerns over the process taken by UNESCO to develop its draft recommendation.
They acknowledged UNESCO and its advisory bodies had limited scope to analyse reports and visit world heritage sites during the coronavirus pandemic.
"However, we underscore the need for intergovernmental and international institutions to continue to apply due process in interactions with state parties," the letter said.
"To that end, any recommendation from the World Heritage Centre and advisory bodies should be based on transparent, extensive and close consultation processes with state parties concerned."
The ambassadors highlighted this was particularly important when the World Heritage Committee was being asked to consider significant decisions, such as an immediate in-danger listing of the Great Barrier Reef.
The letter was signed by ambassadors from Indonesia, Canada, the UK, France, Thailand, Hungary, Poland, Bangladesh, the Philippines, Turkey and Spain.
Australian Associated Press
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