The threats that have been plunging schools around the country into lockdown could be coming from overseas

At least a dozen schools were put into lockdown yesterday because of menacing phone calls -- and several North Island schools received similar threats on Tuesday.

Police say most of the calls referred to an explosive device, suggesting the calls are all coming from the same source.

Assistant Commissioner Bruce O'Brien says the source may be based offshore, and if that's the case police will work with international partners to make sure the caller is punished.

Meanwhile, police are speaking to two young people over completely unrelated threats against Blenheim's Marlborough Girls' College.