UDPATED 12.47PM: The number of methamphetamine users in Christchurch has spiked with a jump of almost 40 per cent since the earthquakes.

A police-funded survey of 800 people arrested across the country shows the largest increase in Christchurch, with numbers in other main centres remained fairly stable.

Lead researcher Chris Wilkins said the increase in construction workers for the earthquake rebuild is behind the increase in demand for the drug.

"Methamphetamine has a long history of use in different industries and activities that require you to be focussed or maintain endurance for a long time."

Manufacturing and Construction Workers Union secretary Phil Yarrell said because traces of weed linger in the body, people are turning to the harder stuff.

"If a person smoked dope two weeks ago, they still show up as positive for drug testing and of course that reflects badly on them, whereas methamphetamine clears from your system in about twenty-four hours," he said.