Planning and fire experts say there may be more than 5,000 buildings in Victoria that contain non-compliant cladding, similar to that used on the Grenfell tower in London.

At least 80 people died earlier this year, when fire ripped through the apartment building covered in polyethylene (PE) cladding.

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The ABC's Four Corners program has revealed Australian suppliers of aluminium-composite cladding knew panels with a PE core were highly flammable, a decade before the deadly London fire.

Flammable cladding has been on the radar of fire and planning authorities in Melbourne since the 2014 fire at the Lacrosse building in Docklands.

In the wake of the Grenfell deaths, the Victorian Government announced it would accelerate the investigation into flammable cladding.

The taskforce is yet to report back, but members have told the ABC there were more than 5,000 buildings in Victoria that were built with non-compliant cladding.

Calls for non-compliant cladding to be removed

Phil Dwyer from the Builders Collective of Australia said it was difficult to tell whether cladding was flammable, but developers would be drawn to the cheaper option.

"It's very, very difficult to tell what's compliant and what's not compliant without destructive testing," he said.

"You can have as much regulation as you like, [but] without enforcement none of it is going to work."

Scott Williams from the Fire Protection Association of Australia agreed.

"This is one of the greatest challenges we've got in the industry because both products actually look the same, feel the same," he said.

"Apart from documentary evidence or actually testing the product there's no other way you can actually tell.

"We just hope that the audit process is thorough - no stone should be left unturned."

Mr Williams said all combustible cladding should be replaced.

"It sends a dangerous message to the industry where we accept something that is non-compliant ... I think there's a matter of principle here as well that we've got to make sure people do what is required, what our codes and standards say."

Experts say it is extremely difficult to tell combustible cladding apart from compliant cladding. ( ABC News: James Glenday )

Work continues to identify high-risk buildings

Metropolitan Fire Brigade Commander Mark Carter agreed the most reliable data suggested there were more than 5,000 buildings in Victoria with non-compliant cladding, including apartments, hotels and hospitals.

However, he said that did not mean the buildings were dangerous.

"That's not to say that a large percentage of those buildings have got combustible facades on," Commander Carter said.

"I think the challenge for the taskforce and the people that are associated with it now is what sort of filters do we apply to identify the high-risk buildings."

An initial audit of 170 buildings in the City of Melbourne, conducted after the Lacrosse fire, found half were non-compliant.

"But out of those we have really landed on around about six or seven out of that initial audit sample that were sort of pushing into the high-risk category," Commander Carter said.

He said high-rise apartments were usually well-protected by fire prevention and sprinkler systems, and buildings between three to eight storeys high were the biggest concern.

"The National Construction Code doesn't require that they have mandatory sprinkler protection," Commander Carter said.

"What we did see with the London fire was it's a recipe for disaster when you don't have a building sprinkler-protected, and you have a combustible facade."

Watch Four Corners' report on flammable cladding on ABC iview.