The Coalition government plans to close the Christmas Island detention centre by July, less than four months after reopening it over fears of an influx of asylum seekers.

Despite Prime Minister Scott Morrison estimating the cost of reopening the detention centre on the island at $1 billion over four years, the budget papers reveal just $185 million has been allocated.

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The government argued the detention centre was required to deal with sick asylum seekers and refugees on Nauru and Manus Island who were approved for medical transfer under legislation passed earlier this year with Labor and the crossbench’s support.

Mr Morrison had also accused the opposition of encouraging people smugglers to restart their trade by supporting the legislation.

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But the budget revealed that the government intends to repeal the medevac legislation if it wins the May election and then re-close the centre by July.

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The facility, which has the capacity to house 250 people, remains empty.

No asylum seekers or refugees have been transferred to Christmas Island from Manus Island or Nauru since the medevac legislation passed in February.

Finance Minister Mathias Cormann defended the prime minister’s previous $1 billion cost estimate, saying the figure was based on advice from intelligence agencies at the time.

Senator Cormann said the government would reverse the “reckless” legislation to limit the number of asylum seekers accessing a medical transfer.

Mr Morrison toured the newly re-opened detention centre in March accompanied by a media contingent to show Australians it was "up to the job".

The trip was criticised by the Opposition as an expensive "waste of time".

An extra $58.6 million has been set aside for ASIO next financial year to boost border security.