A deal between Labor and the Greens to pass the tax hike on superannuation balances above $3 million is all but certain after the Coalition slammed the door on any prospect of becoming involved in negotiations.

As Greens treasury spokesman Nick McKim signalled strong in-principle support for the tax slug ahead of negotiations with Treasurer Jim Chalmers, shadow treasurer Ted O’Brien called the policy “a grave threat to retirement savings, investment, and the property market”.

“Labor’s proposed tax [on unrealised gains] is not a minor adjustment, it is a fundamental and dangerous departure from established taxation principles in Australia,” O’Brien told a Property Council of Australia summit in Canberra on Wednesday.

He said the refusal by Chalmers to index the $3 million was “insidious”.

“Jim Chalmers has argued that Labor’s super tax will only hit 80,000 people,” O’Brien said. “He wants to give the impression that he’s a modern-day Robin Hood, taking from fat-cat investors with multimillion-dollar portfolios to fund good deeds of the government.

“But the way the tax has been designed ensures it will become progressively more punitive and widespread.

“Labor’s super tax is super big and super bad, and it needs to be fought against, especially repugnant features like unrealised capital gains and no indexation.”