Australia Post has sold one of Sydney's most historic buildings, the GPO on Martin Place, to Hong Kong billionaires Robert and Phillip Ng in a secretive deal, despite concerns raised in a heritage report it commissioned last year and never made public.

Australia Post sold the property – subject to final regulatory approval – to Singaporean international property developer Far East, controlled by the Ng brothers, in March for $150 million. It has never announced the deal. 

The deal appears to deliver significant value to Far East, which has plans for new multi-level retail shops that raise questions over use of existing open space and protection of heritage elements.

Fairfax Media has obtained a heritage report commissioned by Australia Post in April last year from Ian Stapleton of Lucas Stapleton & Johnson, the original heritage architects for the redevelopment of the 1 Martin Place complex in 1999, that stresses the benefits of keeping the GPO in Australian ownership.

The report warns: "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 report says.

In addition, 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.

The larger retail floor space would allow a significant increase in rents. 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.

Far East, the largest property developer in Singapore and also linked to Hong Kong's Sino Group, is also planning outdoor, piazza-style dining in Martin Place parallel to the Cenotaph, according to plans seen by Fairfax, as part of a larger revival of the Martin Place precinct.

A development application before the City of Sydney from the existing cafe operator shows the RSL has been engaged in talks concerning the outdoor dining site near the sensitive memorial for several years.

The Ng brothers are the richest Singaporeans with a combined worth estimated at $US10.8 billion.

A story in The Australian in early May said Australia Post planned to sell the GPO, with the proceeds to be used to free up capital to invest in staff and services.

Australia Post had announced a public process last year that it planned to sell all seven state GPOs into a trust structure that would see it maintain a 40 per cent stake in the buildings. The sale was pulled. The Sydney GPO has now been sold subject to final approval as a private deal 100 per cent into foreign hands. 

Australia Post has informed relevant ministers, Communications Minister Mitch Fifield and Finance Minister Mathias Cormann. The sale was approved by the Foreign Investment Review Board last week and is still being considered by Commonwealth Heritage.

In a statement to Fairfax Media, Australia Post confirmed the sale of the building, subject to regulatory approval. It would not publicly identify the buyer. Fairfax Media understands Far East is the buyer 

Australia Post would not say who advised on the sale of the building, noting it was a direct private sale, and said it was not able to disclose more information about the sale due to confidentiality provisions.

The GPO purchase would complete Far East's ownership of the GPO precinct, after buying the adjoining Westin Hotel in 2015 for $445 million. At the time of that purchase, Far East also gained control of two leases struck in the 1990s covering the GPO site - one for the mezzanine, ground and lower ground floor retail areas, another for four upper floors of conference and meeting areas. 

Completing the purchase of the GPO would crystallise a financial windfall by securing control over the building and its leases.

Currently, Far East pays Australia Post a minimum $6.5 million each year for the retail-related lease, and a nominal $1 for the upstairs lease. Two property industry experts spoken to by Fairfax Media said it is possible to calculate from the mooted sale price of $150 million how much each of the leases contributed to the total sale price based on their rent on the leases that run for 99 years.

One noted the sale price appeared fair, adding that buying freehold over a property it held leases over had led to a pricing anomaly.

The retail lease generating $6.5 million a year in rent represents more than $149,999,000 of the sale price, they said. The upper floor lease, generating just $1 a year in rent, represents the sale of four floors of prime CBD real estate for approximately $23. Australia Post declined to respond to the analysis.

In a statement, Australia Post said: "Crucially, the entire building remains heritage protected – an important factor when considering the sale – and the existing post office will continue to operate under a lease that remains in place until 2096. The sale has been approved by the FIRB and is pending approval from Commonwealth Heritage. Australia Post will follow all relevant state and federal laws relating to this proposed sale.

"This transaction will 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."

Australia Post CEO Ahmed Fahour, who was heavily criticised in February by the public and Prime Minister Malcolm Turnbull for his $6 million pay packet, resigned in February and will leave the organisation in July. The deal was struck in March. Australia Post declined to answer if Mr Fahour stood to benefit from the sale, including via bonuses. However, it is understood the sale is not due to be completed until the new financial year.

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

Asked about the Far East plans sighted by Fairfax Media, Ms Lay See Shaw, chief operating officer of Far East Organization's Australian operations, said in a statement: "While the owner is exploring various concepts to enhance the Sydney GPO, no firm proposals or implementation timelines have been set. The owner will be guided by all relevant Federal and NSW Heritage laws and regulations if any development concepts were to be proposed for the building in the future."

The Sydney GPO has occupied the site 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.