Treasury head John Whitehead is again pushing for the tax system to be looked at to counter future pressure on the Government's income.
About 4% of Gross Domestic Product is spent on superannuation and that is expected to double by 2050, when retired people are predicted to make up about 25% of the population.
Spending on health is also expected to double by that time.
Mr Whitehead says there is a risk the Government's revenue base is unsustainable in the medium term.
He believes a capital gains tax could help ease the burden, but says some complications would need to be worked out.