Wellington  - New Zealand will restrict sales of cold and flu medicines in a bid to fight growing use of the illegal drug methamphetamine, Prime Minister John Key announced on Thursday.

New Zealand police say methamphetamine, known as "ice" or "P", is the country's biggest drug problem, responsible for a number of murders and other crimes of violence.

Key said the government will change the law to make cold tablets containing pseudoephedrine, the main ingredient used in the illegal manufacture of P, available only with a doctor's prescription.

"I am determined that we will use the full force of the government's arsenal to fight the problem of P, a seriously addictive drug that is ruining lives," he said.

"P hurts not just users and their families but also law-abiding New Zealanders who suffer from the crime it creates."

Key said police found cold and flu tablets in one-third of the illegal P labs they raided.

Huge numbers of the tablets have been illegally imported from China in recent years and Key said a new customs anti-drug task force would be created to crack down on imports.

The prime minister's chief science advisor, Professor Sir Peter Gluckman, said phenylephrine, which could not be used as a precursor for P, was a generally effective alternative to pseudoephedrine in cold medication.

The Netherlands, Mexico and the American state of Oregon have eliminated over-the-counter sales of medicines containing pseudoephedrine without any unacceptable patient inconvenience, he said. (dpa)