A familiar sight to many, the ageing vessel has plied the waters between Port Kembla and Western Port in Victoria for almost 40 years, carrying steel slabs for owner BlueScope Steel.

The company yesterday confirmed the ship had been sold and would depart for Singapore shortly.

It is understood today's voyage will be the 39-year-old ship's final trip with an Australian crew.

Maritime Union of Australia southern NSW branch secretary Garry Keane said it was a "sad day for Australian shipping".

The 10,584-tonne vessel, built at BHP's Whyalla shipyards in 1973, received a $17 million overhaul last year but was only used for a short time before being taken out of service.

BlueScope announced it would sell the slab-carrier in January after the closure of its Western Port hot strip mill, which meant steel slabs were no longer required.

The company decided to transport hot-rolled coil to the Victorian facility by rail, rather than a "prohibitively costly refit" of the ship.

"So BlueScope's decision to send their coil by rail basically saw the death knell for the last example of Australian shipbuilding on the coast," Mr Keane said.

"It's just a very sad day for us to see an iconic old vessel that's been in this port for 40 years sailing away for the last time under an Australian crew."

Mr Keane understood the ship had been sold for $4 million to $5 million, although BlueScope did not confirm the figure.

He also said the union believed the decision to transport steel by rail was "flawed", a sentiment echoed by Iron Monarch crew members yesterday.

A spokesman for the ship's manager, Teekay Shipping, confirmed the vessel's two crews of 17 would be redeployed to other ships in the company's fleet.

Mr Keane said many could end up further away from their families.

A BlueScope spokesman said the Iron Monarch was decommissioned earlier this year and then sold.

"The Iron Monarch will depart Port Kembla for Singapore when formalities have been completed. We expect this to take place over the next few days".