Increased DNA testing of crime suspects is likely to double the size of the national databank in the next five years.

The expansion stems from a Government plan allowing police to take compulsory samples from anyone they intend to charge with a wide range of offences.

Details and cost estimates remain vague but the Weekend Herald has learned that in its first year the law change is expected to add a further 25,000-27,000 DNA profiles to the national databank, which is held by Environmental Science and Research on behalf of the police.

Even allowing for a gradual drop-off in numbers as police catch repeat offenders, the databank is likely to at least double within five years from 90,000 to 180,000 profiles - about 4 per cent of all New Zealanders.

The first stage of the new law starts in July next year. It will allow police to take DNA samples with a mouth swab or a finger prick from anyone they plan to charge with a "relevant offence" - a long list ranging from murder to illegal possession of a firearm. Police will not have to ask a judge's permission to take the samples or wait for a conviction. They will also be able to take DNA from 14- to 16-year-olds if they are charged with a serious offence and have a criminal record.

The Government wants to extend the law to cover all imprisonable offences by the end of 2011. It predicts the full law change will catch an extra 445 criminals each year, mainly for high-volume crimes such as burglary.

Police are enthusiastic about the new powers. They say nearly all criminals leave some trace which advanced testing can now pick up.

Current DNA matching links almost two out of three crime scene samples to a person on the database and police and forensic scientists are confident they can achieve even better results. But the scheme has run into growing political opposition.

This week Justice Minister Simon Power said he had added two "safety valves" to the bill. The first, which has already been publicised, was to delay the full introduction of the testing, one of National's key campaign promises on law and order. The second was to ask the Justice Ministry to review the whole Criminal Investigations (Bodily Samples) Act 1995, which covers criminal DNA testing, focusing on its operational and cost effectiveness, any amendments made, advances in technology and the act's impact on population groups including Maori.

The review will finish in August 2011, two months before full implementation of the law is due to start.

Meanwhile, Privacy Commissioner Marie Shroff has added her voice to the warnings against passing the bill in its current form. In a letter to Mr Power obtained by the Weekend Herald, she urges the Government to appoint an independent committee to oversee the expanded databank and reassure the public it will not be misused.

In a separate letter she also questions the need to take DNA samples for all imprisonable crimes, which would cover low-level offences such as littering and lighting bonfires.

Treasury officials have dismissed the costings for the scheme as inadequate and Labour claims it will take an extra $20 million a year to pay for DNA collection and testing, plus extra prosecutions and prison beds.

- additional reporting Andrew Laxon