The shift in thinking has implications for rates, which have been rising about 10 per cent a year since 2012 to compensate for cuts to stamp duty.

"Stamp duty will remain a part of our tax system for a considerable time to come," Mr Barr said under questioning at an estimates hearing on Monday.

"It was never the intention to complete stamp duty reform in the first five years and it will take a number of decades. My immediate aim is to have lowest rate in the country. What that means for rates is that increases will be much lower."

Mr Barr said he would make detailed announcements in next year's budget, setting out a five-year tax plan, but "the heaviest of the heavy lifting" on rates was complete and future rates hikes would be lower.

Future tax reform would focus on having the country's lowest stamp duty and helping medium and lower income households into home ownership.

"We will look at future tax reform with those objectives," he said. "There is a policy argument to retain some level of stamp duty over the next couple of decades; we will need to consider at what level."

Pushed by the Liberals' Brendan Smyth on the shift in policy, he said, "The government retains the capacity to be flexible on the exact finish date for stamp duty phasing out", with the eventual abolition to take "a couple of decades at least".

"Ask the government in 2028 what their intentions are for tax reform," he said. "... There are at least four elections between now and that eventual path for stamp duty elimination. I'm not going to quibble over a handful of years, I'll focus on the next five but maintain a long-term objective for its abolition."

The government began tax reform in 2012, aiming to phase out stamp duty on house sales over 20 years and insurance taxes over five years, replacing the revenue with higher rates.

In 2012, the budget said, "the government will abolish conveyance duty over a 20-year period", and the same message has been in every budget since, including the budget delivered by Mr Barr on June 2, which also points to the 20-year abolition of stamp duty. Mr Barr has made much of the ACT's national leadership on the issue.

Mr Smyth, the Liberals' finance spokesman, said later that Mr Barr should say precisely when stamp duty would be phased out.

"Is it going in the 20 years as they promised, or is it now 25 years as he seems to be suggesting, or is it never, when he says a future government may choose to keep stamp duty?" he said, describing it as a "magic tax" that was being abolished but kept growing year by year.

Liberal leader Jeremy Hanson has indicated stamp duty would stay under a Liberal government, with detail to be released closer to the election, and the Liberals looking for "

Asked later for clarification on his plans, Mr Barr said when insurance tax was gone completely in 2016, there was a chance to look at the next tranche of tax reform. Future rates increases would be "smaller each year and progressively much closer to the wage price index".

Stamp duty on a $500,000 home had already been cut by one-third, and first-home buyers and pensioners now paid just $20 in stamp duty on new homes.

An "annually reducing level of stamp duty" would "continue to be collected through until the mid 2030s", he said.