The guided missile frigate is to patrol the northern end of the Gulf and defence officials say its duties will include fighting terrorism in the region, even after the end of the Iraq conflict.

Two other frigates, the Darwin and the Anzac, initially deployed to the Gulf as part of the UN mission to enforce sanctions on Iraq, are scheduled to return next month.

A third ship, the transporter HMAS Kanimbla, is continuing its role as Australian command ship and its involvement in mine-clearance operations near the southern Iraq port of Umm Qasr.

Howard, a staunch supporter of the US campaign to oust the Iraqi regime, thanked the Sydney's 230-strong crew and said it was a proud moment for the Royal Australian Navy as the ship prepared to leave Sydney Harbour.

"It is my hope that during your time, in the not too distant future, the really hostile part of the war to disarm Iraq will have terminated," Howard told the sailors and their families today.

"But that does not mean there will not be very important work following the formal cessation of hostilities.

"It does not mean that the flow of humanitarian support will not need escort support, it will not mean ... we will be free from the possibility of attempts by dissident elements to keep the hostilities going."

Earlier, Defence Minister Robert Hill said it remained the government's intention to return troops home as soon as the war ended, and no decision had been made to commit them to so-called phase four -- the reconstruction of post-war Iraq.

"Our current intention as I said is to bring the forces home at the end of the conflict," Hill told ABC radio.

"Whether we make a contribution in niche areas to phase four ... is yet to be decided."

SPACE.WIRE