Key Points:

A drug trafficker who turned in four of his co-offenders has had 18 months cut from his sentence.

Warren Robert Hadfield was caught at Auckland International Airport in February 2005 bringing 800 grams of pure methamphetamine from Japan.

After being caught he offered to identify his co-accused and the Court of Appeal, in Hadfield's sentence appeal ruling released today, said had he not done so, four others would not have been caught, along with $306,000 found in their possession.

Hadfield was sentenced to six years in jail for importing methamphetamine -- a class A drug.

On appeal he argued he should have received a lesser sentence because of the assistance he gave to police after his own sentencing.

The Court of Appeal agreed, saying of a starting point of 11 years in jail, Hadfield's sentence should be reduced by 60 per cent to 4-1/2 years on the basis of his remorse, early guilty plea, and assistance to the police.

"The additional 1-1/2 years' discount is exclusively for post-sentence assistance," the judgment said.

Crown lawyer Mathew Downs said people giving police assistance should be given credit at sentencing not only for their assistance to date, but for future assistance promised.

That would lower the number of appeals necessary and if a person backtracked on their promise, the Crown could appeal the sentence to get it increased.

The Court of Appeal said that should be the procedure in future cases for people promising assistance to police.

The Court of Appeal said it had been forthright in discussing Hadfield's assistance to the authorities because of an assurance from his lawyer that he had been given a new identity for his protection and would leave New Zealand on his release from prison.