Bill Shorten says the government is "dancing around" the issue of hiking the GST and must come clean on what changes are proposed.
The opposition leader has returned from a tour of Pacific islands to get a first-hand view of inundation due to climate change.
During a visit to Kiribati he donned a headdress and danced with locals after a traditional welcoming ceremony.
However on his return to Australia he got down to brass tacks, saying a higher GST was a "toxic idea".
"What the Liberals should do is come clean," he said.
"They're sort of dancing around the issue - they want a 15 per cent tax, they're scaring people."
He says Australians don't want a rise in the 10 per cent GST and Labor agrees.
Prime Minister Malcolm Turnbull says all options are on the table when it comes to tax, including a broader or higher GST.
But any changes would be taken to an election and be put in the context of broader tax reform involving the states.
