The federal Government is under pressure to come clean on how many asylum seekers are expected to be sent to the Christmas Island detention centre amid reports that the detention centre is set for expansion.

The Opposition has called on the Government to clarify its plans after The Australian newspaper reported yesterday the Government had commissioned a secret audit of the island.

The report said the detainee population at Christmas Island was expected to reach 5000 in the next four years, sparking fears that Christmas Island may soon resemble a penal colony.

Prime Minister Kevin Rudd has denied plans to expand accommodation on the island.

"My advice is that around 2040 people can currently be accommodated on Christmas Island," Rudd said.

"Work is already under way on a new 400-bed additional facility in the main detention centre, which will expand capacity to about 2300 people, and beyond that, no other plans exist."

Attorney-General Robert McClelland reiterated Rudd's statement, saying there were no plans to double accommodation on the island.

However, the denials have failed to quell fears that Christmas Island may soon be expanded to hold double its current capacity.

Opposition immigration spokesman Scott Morrison said the island was already struggling to accommodate detainees.

"We need to remember that these facilities were built, at a stretch, to accommodate 1200 people and Christmas Island, I think, is well beyond its carry capacity at present," he said.

"The answer is not putting in more beds and more tents on Christmas Island. The answer is to address the policies that are attracting the boats."

Morrison believes about 400 new arrivals are coming to Christmas Island each month, compared with 100 who leave. So far this year, 18 boats containing asylum seekers have been intercepted in Australian waters.

Morrison estimates each person spends on average 100 to 110 days on the island.

"At the beginning of this financial year the Government planned for only 200 people to arrive illegally by boat," he said.

"They upgraded that forecast to 1400 in November, and today over 3000 people have arrived in this manner this year alone, which will mean that the budget for running these operations on Christmas Island will have blown out by some $280 to $300 million by the time we get to the end of this year.

"So the Government needs to tell us how big they think this problem is going to be, but more importantly, how they plan to address this plethora of arrivals."