PLANS to amalgamate at least two of the three major Vietnamese detention centres have been shelved at a cost to taxpayers of about $57 million a year. Officially, the Government has blamed the delay on dwindling departures of asylum seekers, but government sources said there were also concerns about the transfer of detainees in the wake of the controversial Whitehead raid on April 7. In January, the Government said plans were in place to close all but two of the camps holding asylum seekers. However, since then the number of people leaving on voluntary repatriation flights has dropped from 2,000 a month to less than 200 a month. The biggest camp expected to close was Tai A Chau, which holds about 7,000 people mostly from southern Vietnam and costs about $57 million a year in operating and infrastructure costs. The largest detention centre in Hong Kong is the Whitehead camp where about 13,000 people are detained. It mostly houses people who fled the northern provinces of Vietnam. A government spokesman said yesterday the camp closure programme was ''always tentative'' and was largely dependent on the pace at which the Vietnamese migrants volunteered to go home. ''The recent slowing down of the rate of repatriation is the reason why we have not moved forward with any plans to close further camps in 1994,'' the spokesman said. However, a government source said the delays were further exacerbated by indecision on how to transfer people from camp to camp without violence. An inquiry was ordered after a raid on April 7 at the Whitehead camp in which more than 1,200 officers in riot gear used tear gas to transfer 1,500 people to the High Island camp in Sai Kung. The report resulting from the inquiry found that officers assaulted a large number of Vietnamese in the operation and that there were several other flaws in the way it was carried out. A Correctional Services Department working group is due to report to the Secretary for Security Alistair Asprey later this month on ways to carry out transfer operations. The department has exerted increasing pressure on the Security Branch to close down camps to free staff for deployment elsewhere. At the height of Vietnamese arrivals in the early 1990s, about 20 camps and halfway houses were operating in the territory. The most recent closure was the Nei Kwu Chau detention centre in January. About 24,800 Vietnamese migrants remain in Hong Kong and must be cleared from the camps by the end of next year, according to an internationally-imposed deadline. However, with present departure numbers and consistent new arrivals, this deadline cannot be met unless a dramatic turnaround takes place. It is expected that the Tai A Chau camp will be the next major facility to close because, as an island detention centre, it is more expensive to maintain and staff. Camp workers have suggested that asylum seekers at Tai A Chau, which is overseen by the Hong Kong Housing Service, may be reluctant to move to the Correctional Services-controlled High Island camp where living conditions were regarded as not as good.