A school's response to a threatening bathroom tag that stated a shooting would happen at the campus the next day, has been criticised after parents and students were not warned.

"There will be a school shooting here on 20/6," were the words written in a bathroom at Hamilton Girls' High School.

The tagging was apparently photographed by some students on Wednesday afternoon, according to the sister of a 13-year-old girl who attends the school.

However, police said they were only alerted by someone at the school to the tagging at 10.45pm that night.

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The sister, who did not want to be named, said an email was sent by the school to parents at 9am on Thursday - the day the shooting was threatened.

It stated graffiti had been found in a toilet block and was being treated "with the highest level of seriousness", and police and the Ministry of Education had been notified.

But the first she knew about it was when a colleague's daughter arrived at her workplace and blurted out that: "There was meant to have been a school shooting at school today mum".

"My first concern: Parents weren't told about the details of the incident," the sister said.

"Why were parents not given the option of sending their kids in or not? The panic it would cause can't really compare to the panic these poor children/teenagers endured all day just waiting for something terrible to happen."

Hamilton Police Senior Sergeant Mike Henwood said police were first alerted to the threatening graffiti at 10.45pm on Wednesday.

"Little bit hard to do something about then," Henwood said. "Once the school effectively opened for business on Thursday we've then looked into it.

"We attended. The writing had been cleaned off. The was some inquiries made into identifying who had done it which were unsuccessful."

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Henwood said police checked social media pages but couldn't find conversations about the threat.

He did not know who had cleaned off the writing. Inquiries are ongoing.

Police then carried out prevention patrols at the school throughout the day including foot patrols, Henwood said.

The email sent by Hamilton Girls' High School to parents after a bathroom tag was found the day before, threatening a school shooting on Thursday. Photo / Supplied
The email sent by Hamilton Girls' High School to parents after a bathroom tag was found the day before, threatening a school shooting on Thursday. Photo / Supplied

Principal Marie Gordon said in a statment to the Herald that teaching staff were notified Wednesday night via email of the vandalism.

"As a school, we have followed advice and guidance from the New Zealand Police and the Ministry of Education.

"We briefed our staff and sent out a message to our parent community on Thursday morning."

In the email sent to parents, Gordon said the school was working with police to manage the situation, "ensuring the safety of all staff and students".

"We ask for your support in shutting down any social media images or conversations regarding the incident."

The email said normal timetables and routines had been adhered to and school would finish at 2.15pm.

In 2016 an Education Review Office stated the school had 1673 pupils, ranging from Year 9 to Year 13.

The sister said there was no assembly to quell rumours on Thursday and she understood some students were frightened, with others staying home.

"My other concern is the communication between students and teachers. Teachers weren't being extra cautious ... our babies, our girls were sitting in class thinking the worst."

The incident comes after another high school student, who has been charged with possession of objectionable material after he allegedly downloaded footage of the Christchurch attacks, discussed how to carry out a mass shooting at his school with his peers.

The 17-year-old has name suppression along with the school, and a judge ordered a psychological assessment at a hearing for the accused in Thames District Court last Friday.