Sophisticated, unknown hackers based outside Australia are believed to be behind the waves of bomb threats sent to schools across the country.

Schools across NSW were targeted for the fourth consecutive school day on Wednesday, including schools on the Central Coast and a northern beaches school, Mona Vale Public, that was also targeted last week.

Since Friday, dozens of schools in NSW, Canberra, Victoria and Queensland have received the same automated phone message warning of a device on school grounds.

Thousands of students and staff members have been evacuated and hundreds of police officers and paramedics diverted.

Schools across Britain, the US, France, Japan, Holland, Norway and Guam have received similar threats, although authorities have not confirmed any links.

There is no indication that terrorist groups such as Islamic State are involved in the hoax and the investigation in NSW is being led by the Fraud and Cybercrime Squad, with no involvement from counter-terrorism officers at this stage.

"The threats appear to come from overseas with no credible evidence they could be carried out here," a statement from NSW Police said.

"There is no evidence these are anything other than hoaxes designed to cause unnecessary disruption and inconvenience ... The purpose and exact source of the threats remains unknown."

It's understood authorities suspect the hoax is the work of mischief-making hackers who have no capability of carrying out attacks but are reasonably sophisticated in their use of technology and the dark web.

Schools appear to have been picked randomly and their locations reveal no patterns.

The phone messages contain no religious, political or ideological references that might link them to a particular terrorist group.

Neil Fergus, chief executive of Intelligent Risks, said the situation puts authorities in a bind. Schools cannot ignore the risk but the mass disruption is exactly what the perpetrators want.

"When you've got an automated voice message like this, at least at this point in time, it has to be taken seriously," he said.

"The fact that somebody has the IT skills to perpetrate it the way they have is a concern because it's quite duplicitous. It's a reasonable cloak to avoid identification, detection and prosecution."

Mr Fergus, who advises governments, businesses and schools on security risk, said the spate of co-ordinated threats was "unprecedented".

He said there was an outside chance it could be the work of a group such as Islamic State but it is likely they have other priorities.

"It's well within their capabilities but it's hard to see what they would obtain out of it other than putting more pressure on themselves to move on to the black web," he said.

"Why they'd waste their resources on this, it's hard to imagine."

He said it was a "clarion call" for schools to reassess their emergency plans.

A small group of international online troublemakers, who asked for students to email them with requests for bomb threats, have claimed responsibility for some of the European and US attacks.

In an email exchange with Fairfax Media, one of the group members said no Australian students had contacted the group and the attacks here were likely to be the work of "copycats".

Using the name Viktor Olyavich, the member said the group had targeted 300 schools in the US, Norway, Britain, France, Japan and Sweden and one member had a "particular bias" for attacking schools in Massachusetts because he lived there before returning home to Iran.

In Melbourne, police are investigating whether several of the threatening phone calls have originated from one local school, Nossal High School.

It's possible a hoaxer hacked into the school's telecommunication service to make it appear as though the calls had come from the school, The Age reported.