Safety investigators are urging a shake up in the oversight of ferries on the increasingly congested waters in and around Circular Quay after a spike in close shaves on Sydney Harbour.

The number of so-called close-quarter incidents involving ferries surged to 30 last year, from five in 2010. However, the actual number could be higher because investigators believe many incidents are not reported to authorities.

A report by the state's Office of Transport Safety Investigations said the booking system for wharves at Circular Quay had increased the complexity of navigating the busy waterway because it allowed commercial services to operate in parallel with scheduled ferries.

Investigators said the allocation of wharves, which resulted in west-bound ferries berthing towards Circular Quay's eastern end, led to ferries crossing paths near congested spots on the water.