The economic impact study, conducted by NRMA Motoring and Services, looked at completing the F6 from St Peters to Loftus with a tunnelled section between Sylvania and Loftus.

The study found the extension would cost about $2.2 billion to build and maintain, but would deliver immediate benefits of almost $1.1 billion.

NRMA deputy president Michael Tynan said the report found the long-term economic benefits would reach almost $3 billion by 2031.

Mr Tynan said the Illawarra community would benefit through growth in both jobs and local industries.

"The net economic benefit of $3 billion would be achieved through greater delivery of goods and services across Australia, particularly from Port Kembla and Port Botany, and the new industries that will grow in the area once the motorway is built," Mr Tynan said.

For motorists, a new extension would mean that travel times between Sydney and Wollongong would be reduced from 70 minutes to just 55 minutes.

Mr Tynan said the NRMA's plan for the future of the F6 made "economic and motoring sense".

"Motorists need their politicians in Sydney and Canberra to show leadership when tackling the problem of Sydney's outdated road network," he said.

"Completing the F6 would be good for Sydney's road network, traffic congestion, road safety, the environment and motorists."

The proposed tunnelled section would run underneath residential areas between Loftus and Port Hacking Rd at Sylvania.

The NRMA report also assessed the cost-benefit ratio for completing the F6 without the tunnel.

While the total cost would be $1.1 billion, the long-term benefits would only be $2.5 billion and traffic would still be forced into residential neighbourhoods.

Opponents of the proposed F6 extension have in the past said that 180 homes, a section of the Royal National Park, a school and a church would be demolished to make way for the freeway, but the NRMA said the tunnel would alleviate those concerns.