One of the schools involved confirmed it had received a bomb threat via an electronic voice message.

Just before 4pm NSW Police advised: "Police operations have concluded at a number of schools across Sydney. Investigations are continuing".

Two hours earlier a police spokeswoman confirmed a number of operations were under way, but said there was "nothing to substantiate that there is anything serious at any of them at this stage".

The schools affected included: Cheltenham Girls High School, Hunters Hill High, Sydney Girls High School, Randwick Girls High, Mosman High, Riverside Girls High, James Ruse Agricultural High, South Sydney High School and Caringbah High School.

"At approximately 1:15pm today [Cheltenham Girls High School] received a bomb threat via an electronic voice message," Cheltenham Girls High School posted to its Facebook page at roughly 3.30pm.

"The students were evacuated to the oval whilst the police conducted a search of the school," the post read.

The NSW Police spokeswoman said the threats were "very low level" and said there was "nothing at all" to suggest the threats were related to terrorism.

It is the second school day in a row that has been disrupted by anonymous threats, after police operations were carried out at several schools across Sydney and Melbourne under similar circumstances.

It is understood some of the schools were in lockdown, while others were evacuated, including Sydneys Girls High School, which is located on Cleveland Street in Surry Hills.

Cleveland Street was closed as part of the operation.

The lockdown had been lifted at Randwick Girls High by 2.30pm, but other schools, including Hunters Hill High School, remained locked down.

"We don't have a time frame at this stage, we are just in lockdown," a Hunters Hill High School staff member said. She said she was not yet able to advise when parents would be able to pick up their children.

Police also attended the state's top school, James Ruse Agricultural High. A staff member said that they searched the premises and had departed before 3pm.

It is understood the operation may have finished at Mosman High, with a witness telling the Herald the police had left the school.

Parents of students at Cheltenham Girls High School reportedly received a text message around 3pm informing them that police had attended the school and deemed it "all clear and safe".

"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," a police statement said.

The NSW Department of Education confirmed it was working with police after "several schools received threats today".

"Each school is taking precautionary measures to ensure the safety of its students and no students are in danger," a department spokeswoman said.

"Police are attending at each location as a precaution. Concerned parents can contact their child's school."

It is understood police will be investigating the source of the threats, which will involve the coordination of several local area commands.

Monday's police operation follows a similar series of events on Friday, when a number of schools across Sydney were evacuated after receiving bomb threats.

Threats were made against schools in Mona Vale on Sydney's northern beaches, Woolooware in Sydney's south, Richmond and Penrith in Sydney's west and Lake Illawarra.

It's understood the schools all received the same threatening electronic phone message between 10.50am and 11.10am.

After sweeping the schools, police deemed the threats a hoax and students had returned to class by 2.30pm.

More to come.