Auckland Police have opened a criminal investigation following information that a firearm's been purchased illegally online.

The firearm was purchased by TV3's Heather du Plessis-Allan using false details.

Inspector Peter Gibson said police are making arrangements for the safe return of the gun.

He said to obtain a firearm online, the buyer must provide the seller with a written order countersigned by police proving they have a current firearms licence.

Inspector Gibson said for anyone to possess a firearm without having the neccessary licence is a criminal offence, and if proven in court, could result in a sentence of up to three months in jail or a fine.