Premier Gladys Berejiklian has washed her hands of the controversial sale of one of the state's most significant heritage buildings, the Sydney GPO, saying she won't be speaking out in favour of keeping the building in public ownership.

And prominent heritage architect and Sydney city councillor Phillip Thalis has joined the chorus of concern over the sale, saying the Prime Minister should intervene.

Fairfax Media revealed last week that Australia Post has agreed to sell one of Sydney's most historic buildings, the GPO on Martin Place, to Singaporean billionaires Robert and Phillip Ng in a secretive $150 million deal, despite concerns raised in a heritage report it commissioned last year and never made public. 

The March sale, subject to final regulatory approval from Commonwealth Heritage, came shortly after Australia Post chief executive Ahmed Fahour resigned in February over a pay controversy that attracted community criticism and a rebuke from the Prime Minister. The sale was never announced. Mr Fahour leaves Australia Post in July.

Asked twice how she personally felt about the sale of one of the state's most significant heritage buildings, Ms Berejilklian would not say, noting: "Certainly when we look at all of our assets at a state level community consideration is one of the things we consider and we always also appreciate the heritage value of so many of our assets." 

Asked if she agreed with the Sydney Lord Mayor Clover Moore who condemned the sale last week, Ms Berejiklian said: "That's her opinion." 

She said she would not be speaking out in favour of the GPO remaining in public ownership.

Australia Post informed its relevant stakeholder federal government ministers, Communications Minister Mitch Fifield and Finance minister Mathias Cormann, of the sale in early May. Neither has spoken out in favour of stopping the sale.

On Saturday, further heritage concerns were raised by Sydney City Councillor Phillip Thalis, himself a heritage architect, who told the ABC "the Prime Minister should be intervening ... this is something of national interest" not just a commercial transaction.

Mr Thalis said the GPO is about more than just bricks and mortar, it's about our cultural heritage.

National Trust NSW branch president Clive Lucas last week described the sale as "scandalous", "outrageous" and "a great tragedy". 

A heritage report commissioned by Australia Post last year but never released warned against selling the GPO, noting: "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. Alternatives to any proposed alienation should be vigorously investigated and should alienation proceed, a high level of mitigation is appropriate," the Lucas Stapleton & Johnson report said.

Sydney GPO has been situated in Martin Place since 1830 and the current building "has defined the heart of Sydney's central business district since 1874", last year's heritage report states. 

"It is a building of outstanding architectural and artistic significance and is arguably the finest example of the Victorian Italian Renaissance Revival style in NSW. Its long public colonnade is a rare and very fine architectural element in Australia and it is the largest and most impressive post office building in NSW and perhaps in Australia," the report states.

Elements of the building are rated as of "exceptional" and "high significance".

Fairfax Media has seen plans that purport to show Far East intends to make significant changes to the internal structure of the GPO, including a row of luxury shops on the ground floor that would extend into the lower ground floor, which is currently open space.

Other plans include filling the void in the middle of the ground floor, allowing for greater cafe access, and removal of columns that were part of the heritage renovation in 1999.

In a statement, Australia Post said: "The proposed sale of Sydney GPO has no impact on the heritage protections covering the site but remains subject to approval by Commonwealth Heritage.

"If the sale proceeds, we will work with the Department of Environment and Energy who administer the Commonwealth Heritage List, and other authorities, to further ensure heritage values are protected."