SYDNEY - Schools in Australia's New South Wales state and Victoria state were placed lockdown on Friday after phone threats sparked police search operations, just days after students returned from their summer holidays.

Approximately seven schools in New South Wales state were on temporary lockdown and searched by police on Friday morning after multiple phone threats that had been received at local, interstate and overseas schools over the past 24 hours.

A New South Wales police spokesman told Xinhua students returned to class in the early afternoon. However, the investigation is ongoing and the authorities are continuing to liaise with the local education department.

"Making such threats is a serious criminal offence and every effort will be made to identify the person or persons responsible," NSW police said.

The search in New South Wales follows the evacuation of multiple schools in Victoria state after similar threats were made via recorded messages on phone calls, however Fairfax Media reported at least one Melbourne school had "received an actual phone call", according to a student's mother.

Both New South Wales and Victorian authorities won't confirm the exact type of threat.

"The schools are being evacuated as a precaution," Victoria police said in statement.

A spokesperson for Victoria's education department told Xinhua the affected schools enacted their emergency management plans which included evacuations after they reported the threats to police earlier on Friday.

"The safety and wellbeing of our students is always our number one priority, and the Department is taking these incidents very seriously," the spokesperson said.