The Maritime Union of Australia said the spill was further proof that Australian shipping must be reformed before "another disaster occurs".

The Magdalene, a 21-year-old Liberian-flagged coal carrier, has been blamed for the spill.

Newcastle Port Corporation said 30 people had worked for five days to clean up the 12-tonne oil spill, which happened last Wednesday.

The National Parks and Wildlife Service helped rescue oil-covered pelicans, which were taken to the RSPCA in Tighes Hill.

"Most of the oil spill was contained on the corner of the Kooragang basin," port corporation chief Gary Webb said.

"We are now focused on residue oil that is being cleaned from rock faces and emerging from under wharves," he said.

The maritime union's national secretary Paddy Crumlin said the Magdalene's crew was "operating under inferior conditions and grossly underpaid to do a highly skilled and demanding job".

"Liberia is notorious for its blood diamonds and the dodgy flag of convenience ships it registers."

Mr Crumlin said the incident showed Australia had not learnt the lessons of the Pasha Bulka grounding in 2007.

He said foreign ships were responsible for all recent shipping accidents that harmed the environment, including the Hong Kong-flagged Pacific Adventurer which spilled oil on Queensland beaches last year.

Mr Webb said a preliminary investigation suggested the Magdalene was "deballasting water to load coal" when the spill happened. Investigations will continue for a few weeks.