SYDNEY - Police in Australia investigating threatening phone calls that led to the evacuation of more than 30 schools in recent days said on Wednesday they suspected the hoax calls originated overseas and were part of a sophisticated hacking system.

The calls, which began last Friday, have made threats of bombs or shooters at schools in at least three states and the Capitol Territory.

Occurring almost daily since, the calls appear to have been automated, stored on servers, and unleashed in batches over the Internet, according to authorities.

The incidents follow similar hoaxes that have led to the evacuations of thousands of school children in Britain, France, the Netherlands and Japan in recent months. Australian authorities did not say whether the threats were linked.

Police in Australia's most populous state, New South Wales, said in a statement that there was no indication that the hoaxes were related to terrorism, and urged media to show restraint to minimise publicity for the perpetrators.

"There is no evidence these are anything other than hoaxes designed to cause unnecessary disruption and inconvenience," the police statement said.

"The threats appear to come from overseas with no credible evidence they could be carried out here."

Australia, one of the largest contributors to the United State-led bombing campaign against the Islamic State militant group in Iraq and Syria, is on heightened alert for attacks by home-grown radicals.

Authorities say they have thwarted a number of potential attacks, although there have been several "lone wolf" assaults, including several involving school-age teens.

Police shot dead a Melbourne teenager in September 2014 after he stabbed two counter-terrorism officers. In October of last year a 15-year-old boy was killed by police after he gunned down, at close range, a police accountant in Sydney.

The recent threats are believed by authorities to involve a fairly high degree of technological sophistication.

James Merlino, Education Minister in Victoria State, said that police were investigating whether some of the calls had been routed through hacked telecommunications equipment at a prestigious high school in a Melbourne suburb.

"It may be that the hacking and the telecommunications are bouncing around the world and landing in this school," Merlino told ABC Radio on Wednesday.

Nearly 4,000 children were evacuated after four secondary schools in Britain received hoax bomb calls last month. No bombs or suspicious objects were found, police said.