Flames broke out in the cargo hold of the Iron Chieftain on Monday morning as crews were unloading dolomite at Port Kembla Harbour.

The blaze on the outer deck was put out later that day, but a "deep-seated fire" continued to burn below and was "proving difficult to access".

But authorities had a breakthrough on Thursday morning after applying high-expansion foam into the cargo hold.

"The strategy appears to have been successful," Fire & Rescue NSW tweeted.

"There are no signs of smoke from the ship and field temperature levels are dropping, indicating the fire may have been extinguished."

A salvage plan is now being developed.

About 100 firefighters remain at the scene, having battled the blaze from the wharf and ship's deck, while tugboats applied water to keep its fuel tanks cool

NSW Fire & Rescue spokesman Norman Buckley said a fire in a ship hold was the "most difficult" a firefighter will battle in their career.

A meeting with the ship's owners, insurance parties and the salvage operations team was scheduled for Thursday.

Fire and Rescue NSW will hold a press conference at 2pm. Watch it live on our Facebook page.

AAP