Methamphetamine is cheaper and easier to buy and there is some evidence of a "cannabis drought", a study based on interviews with people held by police on drug offences.

There are now more detainees who feel dependent on synthetic cannabinoids - particularly in Christchurch - and there's a developing black market for the drugs, manufactured without any safety standards.

The New Zealand Arrestee Drug Use Monitoring study by Massey University has been published by police.

It comes after an election campaign in which drug policy and measures featured prominently - including vows to increase penalties for dealing synthetic drugs, and a push for medicinal cannabis.

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The most recent study interviewed 800 police detainees at four central city police watch houses - Whangarei, Auckland Central, Wellington Central and Christchurch Central - from June to October last year. Urine samples were collected from 202 detainees to confirm the drugs used.

Eighty-five per cent of detainees were men, and the mean age was 29.

• Methamphetamine

The study found increasing use and availability of meth and dropping prices.

Detainees who reported using meth in the past year increased from 26 per cent in 2010 to 38 per cent in 2016, with particular increases in Christchurch and Wellington.

The mean price of a gram of meth dropped from $788 in 2014 to $620 last year.

• Cannabis

The study found some evidence of a "cannabis drought", with the proportion of detainees in Christchurch who used the drug in the past year falling from 81 per cent in 2010 to 66 per cent last year.

Detainees reported difficulty in getting cannabis in Whangarei, Wellington and Christchurch, with the overall mean price for an ounce increasing from $323 in 2011 to $340 last year.

"These findings support anecdotal reports of a "cannabis drought", particularly in the South Island, and may reflect the higher profits available from synthetic cannabinoids and other drugs, and the effectiveness of police cannabis crop eradication operations," the report stated.

• Synthetic cannabinoids

The banning of legal high products in May 2014 reduced the number of detainees reporting use of the drugs, but "appears to have left a residual group of dependent users".

The proportion of detainees who said they felt dependent on the drugs increased from 17 per cent in 2013 to 29 per cent last year. The problem was particularly bad in Christchurch - detainees there who used synthetic cannabinoids and felt dependent on them increased from 24 per cent in 2013 to 47 per cent in 2016.

There is also a developing black market for synthetic cannabinoids, the report found.

• Alcohol

Detainees named three drug types as most responsible for their substance abuse problems - alcohol (78 per cent), meth (33 per cent) and cannabis (32 per cent). Nine per cent named synthetic cannabis.

The mean number of alcoholic drinks consumed on a typical day of drinking was 15.

During the campaign National announced an $82 million package to reduce drug addiction by introducing hardline anti-gang measures and funding 1500 additional drug treatment places in rehabilitation centres and prevention services.

Police will get new powers to search the cars and houses of gang members at any time to check for firearms, and there will be higher penalties for manufacturing and distribution of synthetic cannabinoids.

Finally, compulsory police vetting will be introduced for anyone working at ports, mail centres or airport baggage centres, and drug dogs introduced at domestic airport terminals.

The biggest change to New Zealand's drugs law in recent history will happen if a private member's bill by Green MP Julie Anne Genter is successful.

Likely to be debated next year, it would let people with a qualifying medical condition cultivate, possess or use the cannabis plant and/or products, provided they have the support of a registered medical practitioner.

This would also apply to an immediate relative or other nominated person so they can administer or supply cannabis to the person.

Labour, the Maori Party, and Act have said they will back it at first reading to allow a committee to consider safeguards. NZ First is generally supportive of medicinal cannabis but has not revealed how it will vote.

National has not stated its position, or if it will be a conscience vote for its MPs.