Inadequate public transport in Sydney's outer suburbs is leaving its residents at a significant disadvantage to those closer to the inner city, the nation's top infrastructure adviser says.

Infrastructure Australia has urged the government to accelerate the use of new technology such as on-demand buses to help close the gap, while removing fare penalties for commuters who switch modes of public transport to get to their destinations.

Underscoring the divide between the east and west of Sydney, more than one million people in outer suburbs – extending out from Sutherland, Liverpool and Castle Hill – do not have access to frequent public transport services within a 15-minute walk of their homes.

Residents of Sydney's outer suburbs tend to rely heavily on their cars due to poor access to frequent public transport services. Credit:Michele Mossop

A new report by Infrastructure Australia has warned that outer areas of Australia's largest cities “are being left behind”, experiencing poor service frequency, lower levels of access and longer travel times.