The head of National Trust NSW has slammed the planned $150 million sale of Sydney’s historic GPO building to Singaporean developer Far East Organisation as “scandalous”.

The GPO in Martin Place is widely recognised as one of the most important colonial post offices in the country. It has a deep architectural history. It served as the official centre from which distances from Sydney were taken. And it is the official heart of Sydney’s ‘2000’ postcode.

“I think it’s scandalous that such an important building in such an important city should be sold off in this way,” Clive Lucas, NSW branch president of National Trust, told AAP on Thursday.

“A nation that forgets its history is a nation that has no future.”

Malcolm Gunning, president of the Real Estate Institute of Australia, also told The New Daily he questioned the secrecy around the sale of the building. Australia Post did not reveal the buyer in a statement issued on Wednesday.

“Why wasn’t the GPO put to a public tender?” Mr Gunning asked.

He said there was a probity issue as the building wasn’t publicly sold and he questioned whether Australia Post, a taxpayer-owned building, got value for money.

“I think you could say it doesn’t stand the pub test very well – $150 million for the most prominent building in Martin Place sounds a bit cheap.”

Singaporean Far East Organisation already has ownership of the adjoining Westin Hotel section of the GPO, sold off by Australia Post in 1997 at which time a 99-year lease was put in place for the other half.

Far East bought the hotel for $445 million in 2015.

The sale of the GPO still needs the approval of Commonwealth Heritage.

A spokesperson for the Department of the Environment and Energy, which oversees Commonwealth Heritage, said it “will work with Australia Post through the divestment process”.

In a statement Australia Post said the deal would, “free up valuable capital to invest in our people and in our services to the community, without any impact on the continued operation of and heritage protections covering the Sydney GPO site”.

But a heritage report from Ian Stapleton of Lucas Stapleton & Johnson, the original heritage architects for the redevelopment of the 1 Martin Place complex in 1999, which was commissioned by Australia Post last year, recommended keeping the building in Australian hands.

Fairfax obtained a copy of the report, which warned: “The sale or alienation of the place is considered to be very undesirable.”

“Very substantial loss of significance may occur should any part of the place be sold or otherwise alienated from Australian ownership,” the report said.

“Alternatives to any proposed alienation should be vigorously investigated and should alienation proceed, a high level of mitigation is appropriate.”

The Martin Place GPO was closed in 1980 for safety reasons, before being extensively renovated between 1997 and 1999.

Under the current lease, Australia Post has all control over heritage aspects of the site and control would be returned in 2096.

The building is widely regarded to have great architectural significance, but Far East is reportedly looking to modify the interior of the building, including filling the distinctive void in the middle of the ground floor and removing columns from the 1999 renovation.

Director of City Plan Heritage, Kerime Danis, told The New Daily that taking buildings like the Martin Place GPO out from government hands could make them vulnerable “unless there is a mechanism to make sure the building is looked after”.

But she noted any changes to the interior of the building would need to be approved by the NSW Heritage Council.

A spokesperson from the NSW Office of Environment and Heritage said that, although the GPO is protected under the Heritage Act, they had no opinion or role on the sale.