The state's transport agency has been warned that the amount of highly contaminated sludge dredged from Sydney Harbour for a multibillion-dollar road tunnel and barged to White Bay in the city's inner west could be "more significant" than predicted.

The warning from the Port Authority of NSW comes as the proposed Western Harbour Tunnel faces stiff opposition from schools, councils, residents and community groups.

The vast bulk of nearly 1450 submissions responding to Transport for NSW's plans for the road project are opposed, citing fears about the impact of construction, the loss of parkland and emission stacks to be built for the tunnels.

Highly contaminated spoil will be barged to White Bay in Sydney's inner west.

The project includes twin 6.5-kilometre tunnels from an underground junction for the WestConnex toll road at Rozelle to the Warringah Freeway at North Sydney.