Outgoing Future Fund chairman Peter Costello says the $206 billion sovereign wealth fund should continue growing for another 10 to 20 years, after which it could begin paying cash dividends to the federal government.

Mr Costello, who will step down as chairman in February, said the fund should keep accumulating returns to offset the $900 billion of Commonwealth gross debt and other unfunded government liabilities that are likely to grow as the population ages over coming decades.