A speed limit of 40 kilometres an hour will be extended to cover a larger part of the western edge of Sydney's central business district in an attempt to reduce crashes and pedestrian injuries.

Three years after the area covered by a 40km/h limit in the CBD was increased by about three quarters, the government has incorporated Barangaroo, Dawes Point, Millers Point and parts of Ultimo from August 11 when "high pedestrian zone" signs will be installed.

Centre for Road Safety executive director Bernard Carlon said extending the zone would improve pedestrian safety in areas where more than 90 per cent of daily trips were made on foot.

A reduction in the speed limit in Sydney's CBD has cut the number of pedestrians seriously injured in crashes. Credit:Nick Moir

"A vehicle that hits a pedestrian at 50km/h is twice as likely to cause a fatality as the same vehicle travelling at 40km/h," he said.