The National Trust says the historic Sydney GPO building is "not being protected" after Australia Post finalised the sale to a foreign business venture operated by two Singaporean billionaires.

Federal Environment Minister Josh Frydenberg signed off on the heritage management plan this week, paving the way for the sale of the sandstone building in Martin Place to be finalised. A joint venture between Far East Organisation and Sino Land Company paid about $150 million for the iconic building. The…