Residents have ruled out mounting a court challenge themselves against the approval of the Heathcote Hall development.
Instead, a last ditch appeal to stop the project has been made to Premier Gladys Berejiklian and the Minister for Planning and Public Spaces Rob Stokes.
Ms Berejiklian and Mr Stokes were asked to intervene urgently by either freezing the approval or directing Sutherland Shire Council to mount an appeal.
Phil Targett, a leader of the Save Heathcote East group, said they had received legal advice a 'third party' appeal to the Land and Environment Court would have little chance of success and may cost in excess of $200,000.
Sutherland Shire mayor Carmelo Pesce was given similar advice by senior council officers after the December 13 decision.
Mr Targett said, "In addition, there are matters that exceed the role and authority of the L & E Court that should be addressed by the Premier and Minister Stokes.
"We have asked them I to investigate and fix the broken and dysfunctional DA process."
Mr Targett claimed the DA was effectively approved more than four years ago at a pre-DA meeting between council planning staff and the developer.
The council has previously refuted this claim, saying pre-DA meetings are a normal part of the planning process and the developer receives no assurance a proposal will be approved.
The developer has made changes to the original plans before and during the assessment process.
In a letter to Ms Berejiklian and Mr Stokes on December 29, Mr Targett detailed alleged flaws in the process.
"The Heathcote East community are prepared to be involved in any investigation of this matter and will provide objective and professional details to support all the above matters and identify the validity or otherwise of Sutherland Shire Council and Rural Fire Service decisions," he said.
"A face to face meeting is requested to urgently present all these issues that identify the lack of due process in the assessment of this DA.
"Your urgent response to this request would be appreciated, as the development appeal window will close, we understand, on 10 January 2020."
Planning consultant Brian O'Dowd also wrote a letter, challenging the decision and pointing out the ramifications for future development proposals in E4 zones.
The project includes refurbishment and restoration of the 1887 mansion and gardens and construction of 35 townhouses and 20 apartments in two buildings, with basement parking.