Mounting opposition to the government’s new superannuation tax for balances above $3 million shows how difficult tax reform is, the government’s top economic adviser says.

Treasury secretary Steven Kennedy said the government needed to consider raising additional revenue to reduce the long-term structural budget deficit, which is forecast to remain in the red for the next decade. The cost of the $50 billion National Disability Insurance Scheme would for the first time next year surpass the cost of defence, which was also costing more, he said.