The federal government is also expecting a massive decline in the number of boat refugees over the next few years.

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The midyear budget review, released last week, set aside an extra $2 billion to process refugee claims offshore. The scheme's projected expenses over the next four years are now about $9.5 billion.

However, the government says its annual costs will begin to fall dramatically in about two years, suggesting the current wave of refugees will have dissipated.

Immigration Minister Scott Morrison would not reveal detailed forecasts of asylum-seeker numbers, which rely in part on advice from intelligence agencies.

However, his spokesman confirmed on Sunday that ''illegal boat arrivals were expected to fall from this financial year'' into the future.

An analysis of budget and immigration data suggests that, if the newly expanded refugee centres in Nauru and Papua New Guinea operate continuously at full capacity, and if the average asylum seeker's claim is processed in 143 days (a figure cited previously), the cost to Australian taxpayers per detainee will be about $220,000.

However, if the bureaucracy's method of predicting boat arrivals holds true - it relies on a 10-year rolling average - the current torrent of refugees will slow to a relative trickle (about 5200 arrivals a year), and the average cost per asylum seeker will rise to about $457,000.

The estimates exclude additional expenses of the offshore scheme, such as the costs of customs and naval operations.

Both main political parties back the policy of sending all boat-borne asylum seekers to another country, rather than allowing them to be processed in and settle in Australia.

However, the Coalition blamed this year's soaring costs on Labor, saying it failed when it was in office to adequately fund the scheme.

The midyear budget review said the extra funds allocated this month would ''ensure that 100 per cent of IMAs [illegal maritime arrivals] are processed offshore, whereas the former government had planned to process only one in six offshore and accommodate all others onshore''.

The Immigration Department was detaining 2185 people on Christmas and Cocos islands at the end of November, and 1825 in Nauru and PNG.

However, the government has almost finished expanding the capacity of the two overseas centres to about 4000 detainees.

The Refugee Council says processing an asylum claim offshore costs about five times what it does in Australia.