New Zealand lacks the facilities to forcibly detain people who have severe drug and alcohol addictions, health experts say.

MPs are hearing submissions on the Substance Addiction Bill, which would allow for people with severe addictions to be detained for treatment for up to eight weeks.

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Canterbury District Health Board chief of psychiatry Sue Nightingale told a parliamentary select committee there is a lack of international evidence on the effectiveness of compulsory treatment for addiction.

Dr Nightingale said existing facilities were inadequate to accommodate people detained under the legislation, and there were very few facilities for people who required long-term stays.

"Also we are concerned about the implications for placing people in detoxification programmes, it is very difficult to detain people in hospital beds."

Last month the select committee also heard from a district court judge who questioned whether there was enough treatment centres to cope with demand.

District Court Judge Philip Recordon said the bill did not ensure individual rights were protected, and drug and alcohol addicts might be forced to languish in a police cell before being admitted to secure treatment centres.

Addiction experts have also told the committee addiction services were already underfunded, and adding more patients would inevitably lead to cuts in services in other areas.

The Substance Addiction (Compulsory Assessment and Treatment) Bill is part of Prime Minister John Key's plan to tackle methamphetamine, and the health committee is due to report back in September.