WELLINGTON - Bomb threats telephoned to six New Zealand schools on Wednesday might have come from overseas, a senior New Zealand Police officer said.

Police searched six schools -- four in the North Island and two in the South Island before giving them the all-clear, Superintendent Chris Scahill said in a statement.

"Each call was similar in nature and stated an explosive device had been left on site," said Scahill.

Police searched six schools around New Zealand Wednesday after they received "calls which are of concern", said a New Zealand Police statement.

"Police are liaising with each school and the schools have responded in accordance with their local plans which in some cases included the evacuation of the school," said the statement.

Police are currently assessing the nature of these calls and responding accordingly at each school.

"Police are continuing investigation into the source of these calls, however our inquiries at this stage indicate that the origin of these calls may be overseas," he said.

Police were aware of "very similar incidents", occurring abroad, including in Australia and the United States.

"These have involved recorded voice messages being directed towards schools in these countries," said Scahill.

"Because of this, police are currently investigating whether today's events are the latest in this string of incidents and this will involve liaising with law enforcement agencies overseas."

Radio New Zealand reported that at least two of the schools were evacuated while others sent pupils home early.