NSW police locked down at least six Sydney schools today, after a string of threats were made.

Several schools were evacuated about 2pm and police launched the large-scale operation.

Cleveland Street and South Dowling Street in Moore Park were both closed and police descended on the nearby Sydney's Girls High School.

It is believed police operations also occurred at Riverside Girls High School in Hunters Hill, the nearby Hunters Hill High School and Cheltenham Girls High School and other Sydney schools.

About 4pm, police announced the operations were finished. It is believed no suspicious items were found.

“Police are aware of numerous threats made to schools in the Sydney Metropolitan Region,” a NSW Police Force statement read.

“Investigations are continuing and police are liaising with the Department of Education.

“Police are warning that making such threats is a serious criminal offence and every effort will be made to identify the person or persons responsible.”

Up to six schools have been evacuated. (9NEWS)

It follows a similar incident last week that saw five NSW schools locked down after receiving threats over the phone.

On the same day, schools in the US were also targeted, following a spate of similar calls to schools in the UK, Japan and Europe.

Police said the circumstances between the two evacuations were similar but they are still investigating.

Fairfax Media has reported a group of hackers called Evacuation Squad claimed responsibility via Twitter for some of last week's threats, but claimed not to be involved with the Australian hoax calls.

Last week police searched several schools before determining the calls were a hoax.