Roger Fenwick says there is only one dead end road onto the headland, potentially trapping the proposed home's residents on the steep-sided ridge .

Federal Environment Minister Greg Hunt approved the controversial development just over a week ago, as did the Sydney Harbour Federation Trust which has custody of the former defence site where the facility will be built.

But Mr Fenwick told Fairfax "if it became necessary to evacuate the site, it would be impossible to conduct simultaneous firefighting and evacuation of infirm residents and their carers along this road, and if a tree fell across it  – which is highly likely in a bushfire event – it would be an absolute horror show".

In his formal report he writes: "A proper review must conclude that this is a permanent fatal flaw in the the proposal."

Mr Fenwick's report was commissioned by the Headland Preservation Group, which spearheaded opposition to the proposed 89-bed facility, to be built at a former army site known as 10 Terminal.

The group says allowing commercial residential development at 10 Terminal – which is now part of trust-run parklands – flies in the face of preserving the scenic and historically significant area for the public.

The trust, a federal agency, says leasing of the site will be for 25 years only and will improve the amenity of the area.

The issue has caused tensions with the state government's National Parks and Wildlife Service and Office of the Environment.

A fire consultant hired by the trust, John Travers, recommended the proposed aged care site  could be shielded against "radiant heat"  by building 2 to 3 metre high earth "berms" or barriers around its perimeter, where it adjoins protected bushland on Middle Head.

But the NPWS, which has custody of the adjacent bush areas, has significant concerns about the plan.

Head of the state Environment Office Terry Bailey wrote to Mr Hunt earlier this month, flagging fears that the design of the proposed berms would "act as a dam, blocking natural water flows across the site" and impacting on threatened plant species in the bush areas below.

He also raised concerns about the  "significant visual impact" of the trust's plans for the site. The trust said on Friday that it had imposed conditions that "address the issues raised in Mr Terry Bailey's letter."

Complicating matters is that the NSW Rural Fire Service (RFS) has not yet ticked off on the trust's plans, despite Mr Hunt's approval.  As a federal body, the trust is not legally bound to comply with NSW rules though it has repeatedly stated it would. But  both the trust and the RFS confirmed on Friday they were still in negotiations over the aged care site.

"The trust cannot say it will adhere to state law and then not adhere to it," Mr Fenwick said. "The RFS are to be applauded for their continued scrutiny, and for not rushing to approve this proposal in the face of  pressure from a federal agency."

The state Office of Environment has also asked the trust to be compliant with RFS rules.