THE Finance Minister, Penny Wong, has a blunt warning for delegates attending next week's tax summit: we'll need more tax.

Going beyond instructions from the Treasurer, Wayne Swan, that anything proposed at the summit should be revenue neutral, Senator Wong says, in a paper prepared for delegates, that the government will be tens of billions of dollars short by 2050 unless it raises more.

Finance minister and acting Treasurer Senator Penny Wong at Parliament House Canberra on Monday 26 September 2011. Credit:Andrew Meares

The shortfall by the middle of the century should amount to 2.75 per cent of gross domestic product, which by then will be about $3 trillion in today's dollars.

The reason for the $80 billion shortfall will be the relentless rise of pension payments, which increase with male earnings, a doubling of the number of people over 65, and a quadrupling of aged care spending.