Forty-two people were rescued after their boat smashed into rocky cliffs off Christmas Island last week [Reuters]

The number of asylum seekers believed killed when their boat smashed into rocky cliffs along an Australian island has risen to 48.

The figure was revised as the country's leader gave warning that those still missing may never be found.

Thirty bodies have been recovered since a rickety boat packed with Iraqi, Iranian and Kurdish asylum seekers smashed into the rocks on Christmas Island last week and broke apart in stormy seas.

Forty-two people were rescued from the churning surf, but officials believe many more were swept away in the strong currents, or sank to the ocean floor.

Rescue workers have been interviewing survivors to help determine exactly how many people were on board so they can clarify how many are still missing.

Julia Gillard, the Australian prime minister, said on Monday that those still missing may never be found.

"We are, of course, talking about very rough seas, very rocky and difficult coastline, and so it may be that there are bodies of people who travelled on the boat that are never recovered," she said.

She said that it appears about 90 asylum seekers were on the boat, based on the latest information from the Australian police.

"That does mean, of course, that we are still not able to account for around 18 people on the boat," Gillard said.

"But I do say that we may never know the total number with certainty."

Chris Bowen, the Australian immigration minister, said on Saturday that the search for survivors had switched to a search for bodies in the waters surrounding remote Christmas Island, an Australian territory in the Indian Ocean.

"All [the survivors] on the island are now out of hospital and living on one of the island's three camps," Al Jazeera's Andrew Thomas reported from Christmas Island.

Meanwhile, Australian navy and border control boats have intercepted the first boat to reach the area since last week's shipwreak. The latest boat of asylum seekers were intercepted on Wednesday and brought ashore on Monday, after nearly a week aboard an Australian navy ship. 

An eight year old boy, who lost his family in the wreak, is among them.

"These arrivals are becoming more and more regular on Christmas Island. An average of two boats are intercepted every week," Thomas said.

Australia is a prime destination for asylum seekers hoping to start new lives after escaping from poor, war-ravaged countries.

Asylum seekers who are intercepted by officials are generally sent to a controversial immigration detention centre on Christmas Island, or other detention centres on the Australian mainland.