Eight independent MPs have teamed up to oppose the federal government’s plan to tax unrealised gains on superannuation balances above $3 million, as Treasurer Jim Chalmers struggles to win political support for the biggest revenue-raising measure in his budget.

Six teal MPs, plus two other so-called community independent MPs, jointly announced on Wednesday they oppose the design of Labor’s proposed additional 15 per cent tax on the earnings of larger superannuation balances.