Cronulla and National Park Ferry Cruises operator Carl Rogan said a temporary wharf built by a council contractor failed shortly after passengers stepped off the ferry on Sunday.

He said staff heard the wharf cracking under the weight of passengers, who were moved away before disaster struck.

"We were lucky that they heard the sound coming from the wharf and said stop,'' Mr Rogan said.

"It dropped out behind them.''

A council spokeswoman said Kingston Civil Construction was doing a $1.1 million upgrade of the bridge.

She said the project required the demolition of the old wharf and construction of another.

"As part of the works, Kingston Civil developed a temporary wharf from which the Bundeena ferry would operate,'' the spokeswoman said.

"The bottom landings of the temporary wharf gave way on Sunday afternoon meaning it [was] not possible to make ferry landings on low tide.''

She said the council would investigate what went wrong under the temporary jetty.

Buses had replaced ferries between Cronulla and Bundeena at low tide but regular service has resumed.