Weed and cash accidentally donated to NZ charity shop

Two teenagers mistakenly left a knapsack containing marijuana and cash at a donation area outside a New Zealand charity shop before returning in a panic to retrieve the bag.

A volunteer at the store noticed a pungent aroma before opening the knapsack and finding plastic bags of marijuana amounting to 43.2 grams and $NZ3700 ($A3100) in banknotes, New Zealand Police said in a statement on Monday.

Using marijuana recreationally and selling the drug are illegal in NZ.

Some medical use is allowed with a doctor's prescription.

Police said the episode happened on February 18 in the Southland region of the South Island.

The boy and girl had left the knapsack outside the shop as they waited for their vehicle to be serviced at a nearby workshop, the police statement said.

Officers were called after the agitated teens returned to the store.

Police found an air pistol - which those younger than 18 can't possess without a license and adult supervision - a police scanner and more cash in the teens' car.

It's not illegal to have a police scanner in NZ, but distributing or acting on information heard is a crime.

Police didn't divulge details about any charges the arrested teens faced or if they had appeared in court.

NZ has strict rules about what can be reported from cases before the Youth Court, which usually hears cases of alleged offenders younger than 17.