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.

NSW Premier Gladys Berejiklian won't be speaking out to stop the sale of the Sydney GPO. Credit:Jessica Hromas

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.