In a rare event for the Great Western Highway, a section is about to have its speed limit increased.

Transport for NSW says the speed limit will increase from today at Forty Bends, near Lithgow.

Transport for NSW western region director Alistair Lunn said the existing 80 kilometre an hour speed zone will be increased to 90km/h from around 1.5 kilometres east of McKanes Falls Road, just outside the Lithgow urban area.

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"Transport for NSW completed a speed zone review in consultation with local council, NSW Police and Regional Transport and Roads Minister Paul Toole," Mr Lunn said

"The review was conducted in accordance with the NSW Speed Zoning Guidelines, and considered a number of factors including road environment, usage, and traffic mix."

Mr Lunn said the Great Western Highway upgrade at Forty Bends, completed in October 2017, involved realigning the road to improve safety along the curves, intersection upgrades, a centre median barrier and installing new concrete pavement to lessen the risk of black ice forming.

"This upgrade increased the permissible speed along the curves and decreased the number of conflict points, resulting in a road suitable for a higher speed limit," he said.

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Mr Lunn said new speed limit signs will be installed today between 7am and 5pm, with traffic control and a reduced speed limit in place for the safety of motorists and workers.

Electronic message signs on approach to this section of road will also alert road users to the speed limit reduction, he said.

Transport for NSW says the speed limit will eventually rise to 100 kilometres on the Great Western Highway through Hartley and to the base of Victoria Pass as part of the NSW Government's highway duplication project.

The government has committed $2.5 billion so far to the duplication of the highway from Katoomba to Lithgow, which will include tunnels under Blackheath and Mount Victoria.