A CUSTOMS border protection vessel was forced to rescue 40 asylum seekers after it tore off the bow of their rickety timber boat about 20km from Christmas Island.

The people were transferred from their boat by the Customs crew late on Friday, but were not delivered to the island until Sunday afternoon.

According to sources the suspected illegal vessel was detected by radar about 30km from the island on Friday afternoon and the passengers and crew were issued with a verbal warning about their final destination being Manus Island or Nauru and not Australia.

When they refused to turn around the Customs crew boarded the boat at about 6pm when it was just 20km from the island.

The Customs transit attachment took command of the vessel and turned it back towards Indonesian waters.

About three hours later the engine stopped and could not be restarted so a tow rope was attached.

By midnight the rickety timber boat had suffered terminal damage and began sinking so the asylum seekers were transferred to the Customs vessel.

The decision to move them to Christmas Island was not taken until about midnight on Saturday and they arrived at the island at lunch time on Sunday.

Various government officials, including Immigration Minister Scott Morrison and the commander of the joint agency task force Lieutenant General Angus Campbell, refused to shed any light on the drama or why it took 36 hours to transfer the people to Christmas Island.

Mr Morrison batted away several questions on the matter in Parliament and General Campbell said he would continue to provide only scant information to the public about "on water matters’’ to avoid feeding the people smugglers, but he did reveal that 13 boats carrying 707 people had arrived since the operation began.

He also told Senators that he would only provide information at his weekly Friday update.

Labor Senator Kim Carr reminded the general of his responsibility to answer questions under Senate Standing orders.

“It is not appropriate to be giving incidental advice of arrivals,” General Campbell said.

“I can assure the committee ... where a serious [loss of life] incident arises they are reported at the time.”

The government’s secrecy over the asylum seeker issue has reached the point where Defence personnel in Darwin were ordered by their commander Air Commodore Ken Watson to avoid sending any emails on border protection matters.

Immigration Minister Scott Morrison refused to confirm or deny the sinking and he ordered all departmental officials not to answer any questions about the matter.

Meanwhile the cost of detaining asylum seekers in Australia’s Nauru and Manus Island detention centres is set to blow out beyond almost $1 billion in this financial year.

The federal government will have to find extra cash in the midyear budget review to keep the offshore detention centres running on Nauru and Manus Island, a Senate committee has heard.

Immigration Department secretary Martin Bowles told the estimates hearing the current budget allocation of $900 million would not last until the end of 2013/14.

“We are looking at needing more money,” he told the Senate.