The government took the unusual step of releasing the stamp duty cuts expected in Tuesday's ACT budget a week early, although they won't take effect until July 1.

Ordinarily the cuts are released and take effect on budget day to prevent distortions in the market.

The incoming changes to the First Home Buyer Concession Scheme also mean buyers with a household income of less than $160,000 won't have to pay stamp duty.

However, anyone buying a home under $470,000 would be $7000 better off under the current scheme, as they would pay no stamp duty and get the grant.

Those who buy the median $670,000 house will save $1108 if they wait to buy until after July 1, while those who buy the median $440,000 unit will save $580 if they delay.

Treasurer Andrew Barr's spokeswoman said the projected stamp duty reductions until 2021-22 were already foreshadowed in last year's budget, as were the changes to the first home buyers scheme.

She said the determination simply gave effect to the rates that had previously been announced, and did not constitute an early release.

"Traditionally, new stamp duty rates have commenced on the day after the budget. This is because prior to the tax reform, stamp duty rates for the coming year weren't announced until budget day, and commencing the new rates immediately avoided any potential distortionary impacts," she said.

"Under the tax reform program, stamp duty rates are being announced in five year phases, so the rates for each year are known well in advance - several years in advance for current rates. Therefore it is no longer necessary for duty rates to take effect on the day after the budget."

She said the territory government would continue announcing stamp duty rates well in advance of the start of each stage of tax reform, with an update on July 1 each year.

Stamp duty is being progressively reduced as land taxes and rates increase, as part of the government's 20-year taxation reform.

The government will also fix a mistake in calculations made by the ACT Revenue Office. This mistake could have allowed some home owners to save thousands of dollars more on stamp duty than they should have.

A separate determination laying out the stamp duty rates for the 2019-20 financial year miscalculated the base amount payable in the $500,000 to $750,000 bracket - $8000 instead of $11,400.

The error meant buyers who paid $499,000 for a property would pay $11,366 in stamp duty, but those who bought for $500,000 would pay $8000.

It also meant the base rates for respective brackets were wrong.

The charge for a $750,000 property would have been $18,800 instead of $22,200, or $33,550 for a $1 million property instead of $36,950 had the mistake gone unnoticed.

A spokeswoman for Mr Barr said it was an "inadvertent error" from the Revenue Office, which would be fixed before the new rates kicked in.

Correction: An earlier version of this article incorrectly said buyers would save on stamp duty if they delayed settlement until after July 1. In fact stamp duty is calculated when contracts are exchanged.