Victoria's peak business lobby group is warning Melbourne will not cope without a third airport if the population continues to expand as rapidly as forecast.

The Victorian Chamber of Commerce and Industry says Melbourne's population could increase to as many as 8 million people by 2050.

Spokesman Chris James says action must be taken.

He says suitable options for a third airport include Tooradin or Tyabb, near Westernport Bay, or West or South Gippsland.

"Melbourne's population is anticipated to overtake Sydney's in the next 20 years, and we anticipate that Melbourne Airport - Tullamarine - will be Australia's largest international airport," he said.

"It would be a good idea if we had two domestic airports either side of Melbourne. We've already got one at Avalon; we will need one on the south-east as well."