Home buyers in NSW are being promised cheaper stamp duty under what’s being spruiked as the biggest change to the tax in 30 years.

From July, the seven price brackets that determine how much stamp duty is paid will rise in line with inflation, the government announced on Monday.

But the move, announced four months out from the state election, has been branded by some as inadequate.

Greens MP Justin Field said the policy only “tinkered around the edges” of the problem.

“If the Berejiklian government was serious about housing affordability, it would be capping rent increases to CPI and engaging in genuine property tax reform in NSW,” Mr Field said in a statement.

“It does nothing to address the housing affordability issues in NSW.”

Premier Gladys Berejiklian conceded the changes would currently only save homebuyers a few hundred dollars, but predicted it would increase into the future.

“Whilst today the savings are in the hundreds, in the future the savings will be in the thousands,” Ms Berejiklian told reporters.

“We’re making sure that as the time goes on the savings will increase.”

Opposition treasury spokesman Ryan Park said Labor would not support the “ludicrous change”.

He questioned the timing of the announcement.

“If this were so important it would have happened at the beginning of the property boom, not the end the property boom.”

NSW Treasurer Dominic Perrottet called it the “most significant reform in a generation”, noting the brackets had been largely unchanged since 1986 – when the median house price in Sydney was $100,000.

It’s now more than $1 million.

“Whether you are a first homebuyer, a downsizer or upgrading to the family home you will ultimately benefit as a result of this reform,” he said in a statement.

“Pegging stamp duty to CPI will reduce the tax burden on homebuyers allowing them to put more money towards a deposit.”

Stamp duty makes up about a quarter of tax revenue in NSW but is on the decline.

The government collected about $8.7 billion in 2017/18 – $1 billion less than it previously forecast.

It will take $5.5 billion hit over the three years to 2020/21, partly because of new exemptions for first home buyers.