The Australian Maritime and Safety Authority has abandoned a search for at least 55 asylum seekers who drowned after their vessel sank off the coast of Christmas Island.

The search, which began late on Wednesday, was called off last night after medical advice that no-one could have survived in the ocean for that long.

"The authorities ... will determine whether the search resumes but look it looks very, very, very unlikely and that’s just an awful tragedy," Immigration Minister Brendan O'Connor said this morning.

Another 78 people, believed to be Sri Lankan asylum seekers, arrived on Christmas Island yesterday afternoon after a British merchant vessel came to their rescue.

Boat first spotted on Wednesday; first bodies seen on Saturday

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Listen Duration: 3 minutes 17 seconds 3 m 17 s Jo Jarvis reports on the abandoned search Download 1.5 MB

The boat which sank was first spotted by a Royal Australian Air Force aircraft late on Wednesday afternoon.

Home Affairs Minister Jason Clare says the vessel did not appear to be in distress, but despite Customs aircraft sighting an upturned hull on Friday, it was not until Saturday afternoon that dead bodies were seen floating in the water.

Last night an AMSA spokeswoman said that while 55 men, women and children were originally spotted onboard, it is possible that more were below deck when the boat sank.

The three-day search for survivors involved a large air and sea operation coordinated by AMSA and involving up to three vessels, including HMAS Warramunga, and five aircraft.

Thirteen bodies have been spotted in the water so far, but none have been recovered.

How the tragedy unfolded (times in AEST) On Wednesday afternoon an Air Force aircraft spotted the vessel 28 nautical miles north-west of Christmas Island. At least 55 people were on board. No distress signal was received.

On Wednesday afternoon an Air Force aircraft spotted the vessel 28 nautical miles north-west of Christmas Island. At least 55 people were on board. No distress signal was received. HMAS Warramunga was sent to the location. It arrived at 1:30am on Thursday, but did not find the vessel.

HMAS Warramunga was sent to the location. It arrived at 1:30am on Thursday, but did not find the vessel. The search continued throughout Thursday and Friday, with the help of aircraft.

The search continued throughout Thursday and Friday, with the help of aircraft. Customs asked the AMSA to help with the search. Two nearby merchant ships volunteered to join.

Customs asked the AMSA to help with the search. Two nearby merchant ships volunteered to join. The boat's submerged hull was spotted at 3:00pm on Friday by an Air Force aircraft about 65 nautical miles north-west of Christmas Island.

The boat's submerged hull was spotted at 3:00pm on Friday by an Air Force aircraft about 65 nautical miles north-west of Christmas Island. HMAS Warramunga arrived at the location at 5:00pm on Friday. The hull could not be located but debris including life jackets were found.

HMAS Warramunga arrived at the location at 5:00pm on Friday. The hull could not be located but debris including life jackets were found. An aircraft spotted a body in the water at 8:45pm on Friday, but Warramunga could not find the body.

An aircraft spotted a body in the water at 8:45pm on Friday, but Warramunga could not find the body. At 3.00pm on Saturday, Customs planes spotted at least nine bodies.

At 3.00pm on Saturday, Customs planes spotted at least nine bodies. More bodies have since been spotted, but AMSA has called off the search.

Mr Clare says those on board are believed to have been mostly adult men, but there were also a small number of women and children.

"This is another terrible tragedy. Another terrible reminder how dangerous these journeys are," Mr Clare said.

The minister would not be drawn on the question of who is to blame but has foreshadowed a full review of the incidents after the operation is complete.

"We want to stop people getting here on boats," he said.

"At the completion of this search, this will be [the] subject of a full review, as is standard practice."

Refugee advocate calls for full transparency

Refugee advocate Ian Rintoul says he wants an investigation into why a search was not mounted from the moment the RAAF aircraft spotted the stationary boat.

"I think we do need a full inquiry and complete transparency about what information AMSA had, when it had that information, and why, if there was an indication or even a question that there could be a boat in [trouble], why an immediate search wasn't mounted at that time,' he said.

Greens say 'harsh' asylum policy not working

Greens Senator Sarah Hanson-Young says the tragedy shows Australia's asylum policies are not working.

"We have the harshest policy that this country has ever had on asylum seekers at the moment, and yet people are still coming," she told ABC News 24.

The vessel capsized about 65 nautical miles north-west of Christmas Island. ( ABC News )

"It's the only option they believe they have to seek safety and they are so desperate, they take those dangerous boat journeys.

"We have to find and be more insistent on providing people a safer alternative.

"Until we start responding to it as a humanitarian issue, not one of punishment, not one of deterring people from seeking asylum, it's going to make it very difficult to offer an approach that is humanitarian and humane."

British oil tanker picks up 78 asylum seekers

As the tragedy unfolded, HMAS Warramunga was again deployed yesterday to help another boat-load of asylum seekers north-west of Christmas Island.

That boat has been located and all on board are safe.

As the rescue was underway an oil tanker, the MT British Curlew, transported 78 asylum seekers to Christmas Island after their boat sank 500 nautical miles west of Cocos Island.

In the recent weeks there has been a spike in the number of boats coming to Australia.

Two other asylum seeker boats, carrying almost 200 people have been intercepted since Friday.