It's now feared more than 50 asylum seekers drowned when their wooden boat smashed onto rocks on Christmas Island yesterday.

Immigration Minister Chris Bowen says there could have been up to 100 passengers aboard the Indonesian fishing vessel when it broke up in rough seas.

The search for survivors will continue at first light, but so far just 44 people have been rescued.

As the official death toll rose to 28 this morning, questions were being asked about how the boat was allowed to reach Christmas Island without being intercepted.

Refugee advocates and the Australian Greens have called for an inquiry.

Former diplomat Tony Kevin, who has written about the 2001 sinking of a asylum seeker boat in which 353 people died, says the radar system used by Border Protection Command and the navy is "brilliant".

"I can't believe this was allowed to happen," Mr Kevin told ABC Television.

"Why was a boat allowed to come into the coast and crash when we have the radar and the interception capabilities to make that absolutely impossible."

Mr Kevin said that question had to be asked of Prime Minister Julia Gillard and Home Affairs Minister Brendan O'Connor and shouldn't be hand-balled to the West Australian coroner's court.

All the evidence to date suggested yesterday's tragedy "was preventable" and there had been "apparent negligence", he said.

The federal government confirmed today at least 28 people, including children, died when their boat sank near Flying Fish Cove.

There are 44 confirmed survivors, including 11 children.

Mr Bowen says it's not clear how many people were on the vessel.

"People who have survived say there were between 70 and 100," he told Melbourne MTR.

"But we really don't know and we probably never will."

The immigration minister said he understood the boat wasn't tracked from Indonesia.

Mr O'Connor, who is on Christmas Island, earlier said "we'll have to continue the search before we can determine precisely the number (on board)".

He told ABC Radio some of the asylum seekers were from Iraq and survivors were being interviewed.

A temporary mortuary has been established on Christmas Island.

Six survivors have been taken to hospital while the others have been transferred to the Christmas Island detention centre.

Two seriously injured women have been flown to Perth for urgent treatment.

The Australian Greens say all survivors should be taken to the mainland.

Greens immigration spokeswoman Sarah Hanson-Young also wants the government to launch an inquiry into the disaster.

"We know there is incredible surveillance and sharing of information between Australia and Indonesia," she told ABC Television.

"As the inquiry rolls out ... hopefully we'll start to get some answers (about) who knew what, when and how this happened."

Pamela Curr from the Asylum Seeker Resource Centre has backed the call for an inquiry.

"If indeed they (authorities) did know the boat was coming, which is highly likely, why didn't someone stop it?" she told ABC Television.

Ms Gillard has cut short a holiday to deal with the crisis and will be given a briefing on the situation, along with Mr Bowen, later on Thursday morning.

Acting Opposition Leader Julie Bishop says border protection policies are likely to be debated in the days and weeks to come.

"(But) it's not a question of political-point scoring today," she told ABC Television.