A serious fire on a CSL bulker in Australia probably started on the vessel's conveyor belt, says the fire service in Port Kembla.

All 20 crew were evacuated from the 51,000-dwt Iron Chieftain (built 1993) on Monday.

The Illawarra Mercury cited fire authorities as saying the incident most likely began on the belt before spreading to a 12,000-cbm pile of gravel-like material below deck.