An environmental assessment, detailing the plans to use controlled explosions to bring down the stack, was placed on public display on April 30.

At the time Port Kembla Copper (PKC), which owns the site, told the Mercury it could cost up to $20 million to repair and preserve the stack and raised questions about who would be responsible for upkeep and safety into the future.

But PKC has now been told by the Department of Planning that it needs to submit a detailed heritage impact assessment before the demolition application can be determined.

Two other heritage listed items on the site - the Assay office building and a smaller brick chimney stack - are being retained.

It is understood that a full heritage assessment by PKC could cost up to $100,000 and delay the approval process by up to three months.

In a council submission to the Department of Planning, released to the Mercury by the council, general manager David Farmer said the demolition proposal should "not be approved" until a heritage assessment was completed.

"Whilst not a listed heritage item, it is clear that the 200m high stack proposed for demolition is of heritage significance," Mr Farmer wrote.

"The stack has become a grand and highly visible and evocative symbol of the importance of the industry of Port Kembla to the development of Wollongong and the Illawarra."

The submission also suggests the "archaeological potential" of the site and risk of disturbing relics has not been considered.

The close proximity of the site to heritage listed buildings on the former Port Kembla Public School site is also raised.

The council declined to comment on the submission as the Department of Planning was still assessing the application.

However, the Mercury understands the heritage of the stack has not previously been investigated and in July, the council resolved to undertake a major review of the city's outdated heritage schedule.