Asylum boat arrivals strain Australian detention centre capacities Asylum seekers arrive at Christmas Island last month. Picture: Sharon Tisdale Source: PerthNow THE arrivals of almost 900 asylum seekers in the first four days of May have pushed Christmas Island beyond capacity. Another eight mainland detention centres are overcrowded, after the occupants of boats which have sailed reached close to the Northern Territory in recent days were taken to Darwin centres. A group of 160 asylum seekers intercepted Friday on an inter island Indonesian ferry, and 109 people found on another boat on Saturday, were taken to Darwin. The Immigration Department denied the transfers to the Northern Territory instead of Christmas Island was because the detention population there reached 2962 on Friday. Capacity across five detention facilities on Christmas Island is normally 1094, but can be stretched to 2078, according to the department's accommodation capacity figures. The government, which had predicted asylum costs would begin falling after this year, is expected to revise its forecasts in next week's Budget. It had predicted 12,000 people would arrive this financial year, but 20,211 have come so far in 2012-13, including record months in March and April. Opposition immigration spokesman Scott Morrison claimed the blowout in costs over the forward estimates if arrivals continued at the current rate would be $5 billion. He said the influx since the government announced a return to Manus Island and Nauru in August last year had created a backlog in processing. "We still understand that even as today very few if anyone who has arrived since 14 August last year have actually been processed," he said. The scale of the backlog this government is building up on a daily basis will have an economic cost that will cost for some time in working through that dreadful legacy. In addition to the Christmas Island detainees, there are just over 10,000 asylum seekers in the community on bridging visas, 5892 in mainland detention centres and 2840 in community detention. Just 420 asylum seekers are on Nauru and 294 at Manus Island. About 40,000 asylum seekers have arrived on Australian shores since Labor came to power five-and-half years ago. Sydney immigration residential housing, Yongah Hill in WA, Northern Immigration Detention Centre in the NT, Berrimah House and Darwin Airport Lodge, Brisbane Immigration Transit Centre, Scherger and Inverbrackie in SA are all over capacity. Meanwhile, Customs has found its surveillance efforts were appropriate despite an asylum vessel reaching Geraldton undetected last month. An internal review has found the Sri Lankans on board the Deutsche Bank donated boat were intending to travel to New Zealand, but were thwarted by weather and safety concerns.