The cost of a state Liberal election promise to give Tasmanian seniors free access to national parks has blown out by hundreds of thousands of dollars because a section of the constitution forced the State Government to extend the offer to all Australian seniors.

Key points: Before the 2018 election, the Liberals promised free national parks access for one year for Tasmanians with a seniors card

Before the 2018 election, the Liberals promised free national parks access for one year for Tasmanians with a seniors card Section 117 of the constitution protects against discrimination on the basis of state of residence, so the scheme has had to be expanded to all senior Australians

Section 117 of the constitution protects against discrimination on the basis of state of residence, so the scheme has had to be expanded to all senior Australians The scheme was meant to cost $750,000 over three years, but foregone revenue is already $1.5 million after just one year

Before the March 2018 election, the Liberals promised that 100,000 Tasmanians with a seniors card would have free entry to the state's national parks for one year, and then receive a 50 per cent discount, valued at $38 a year.

The policy was meant to cost $750,000 over three years.

The scheme aimed to ensure wilderness areas were accessible to Tasmanians and encourage seniors to be active.

But the head of Tasmania's Department of Primary Industries, Parks, Water and Environment (DPIPWE) John Whittington has told a budget estimates hearing that the scheme had to be expanded to cover all Australian seniors because section 117 of the constitution provides protection against discrimination on the basis of state of residence.

"We can't differentially charge non-Tasmanians a different rate to Tasmanians," Dr Whittington said. "We couldn't have a post code-based pricing system."

The Premier said the park offer was "something we can boast about". ( ABC Radio Hobart: Carol Rääbus )

After questions from Labor MP Alison Standen, the committee heard that 19,700 Australians aged over 60 had taken up the free park entry pass since July 2018.

Premier Will Hodgman told the hearing the policy had already cost $1.5 million in foregone revenue — double what the Government had expected the scheme to cost over three years.

Mr Hodgman, who is also the Parks Minister, said he estimated the impact would be $330,000 per annum in ongoing years.

"Given the expansion of the offer to all Australian senior card holders, it's certainly something we can boast about, that you're able to get such a great concession here," Mr Hodgman said.

Mr Hodgman said the majority of parks visitors were interstate visitors, most of whom would currently be purchasing a $60 visitor pass.

"[With] the offer of free entry and then reduced annual pass fee in subsequent years, it's anticipated there will be a significant reduction in the sale of holiday passes to the over 60s, resulting in a decrease in revenue," he said.

The Government is currently reviewing park entry fees, which the Premier said had remained unchanged since 2009.