A former maths teacher at St Kentigern College has been censured for an Instagram chat with students discussing sex and drugs.

The teacher, Dr Edward Coad, discussed prostitutes, genitals size, drugs and binge drinking in an Instagram conversation with a group of Year 12 students at St Kentigern a few days after leaving to take up a new job as head of maths at Northern Southland College in Lumsden.

The Teaching Council's complaints investigation committee said the themes of the chat "included sexual innuendo", topics of an explicit sexual nature, reference to sex "outside of the immediate family", binge drinking and the drug MDMA.

He resigned from the Southland college two months after the Instagram chat came to light and now describes himself as "retired with a bit of derivative trading in the mornings".

Coad has a doctorate in physics from Cambridge University and had a career in financial trading in Britain before training as a teacher at Auckland University in 2015-16.

He worked at St Kentigern College in Pakuranga from June 2018 until August 23, 2019.

At some time between August 23 and August 28 in 2019, while he was travelling alone to Southland leaving his family in Auckland, he was invited to join an Instagram chat with what he described as "most of the class" in his Year 12 group at St Kentigern.

Excerpts from the chat disclose that a student commented: "Sir is a functioning alcoholic."

Coad posted a photo. The content is not disclosed but he followed up with the comment" "Another 8 to get down... need to find some additives."

A student said: "I like sirs influence. He lives life without f----s given."

Coad responded by posting a photo of his cats with the words, "These are too."

A student said: "Sir I need you to come back. Who else is gonna drive me to my prostrate [sic] exam..."

Coad replied: "... What is this s**t? Best time at 8pm?? You come in wrecked from too much sleep. Prostrate exams start at 40, mark my word."

Another student asked: "What's prostrate."

Coad replied: "Male g spot."

The students discussed prostate examinations among themselves. One said his uncle had shown him how to do it and another student asked Coad whether his uncle had shown him too.

Coad replied: "Nope... a girl in New York.... And I didn't pay for it, before you ask."

Students asked him whether his children were with him. He replied: "Nope... solo. You know what that means."

A student replied: "Hookers and coke."

Coad replied: "I know. NZ short on both, and both cost $350."

A student said: "Sir my uncle became my aunty and now he/she is a prostitute so hey I can get you in contact."

Coad responded: "I just got 20% because the guy thought I was local."

Later, Coad told the students he had received a welcome package from his new school.

A student said: "Coke, molly and pills..."

Coad replied: "No... tim tams, baked beans, tea and coffee."

Students told Coad how much they loved him, discussed genital sizes and asked him "What you have."

Coad responded, apparently referring to his new school: "18 teachers at the school so tiny."

Later, still discussing his new school, he said: "Teachers are about $50,000 better off here. Disposable income etc."

A student said: "Their loss."

Coad responded: "Is it? MDMA should be under $200 here."

A deputy principal at St Kentigern made a mandatory report to the Teaching Council about the Instagram chat on September 9, 2019.

Coad resigned from his job at Northern Southland College in December.

The Teachers Disciplinary Tribunal said Coad "accepted he engaged in the Instagram conversation with his former students and expressed regret and embarrassment at his involvement".

The complaints assessment committee accepted that Coad did not initiate the conversation but said he "willingly participated" when he should have "removed himself from the conversation immediately".

Coad told the tribunal that the interaction was after he had left the school, was in a group setting via a digital platform, and was never in person or on a one-to-one basis.

He said he never approached individual students via social media and didn't initiate this group chat or group, but was invited while in transit to the South Island.

The tribunal found that Coad's decision to engage in the chat, and what he said in it, were "extremely silly" and amounted to "serious misconduct".

"The role of a teacher in maintaining appropriate standards and boundaries does not end when he or she leaves the school," it said.

It decided not to cancel Coad's teacher registration because the Instagram chat was "part of a group conversation and did not involve him forming an inappropriate relationship with one student".

Instead, it censured him and made an order that if he returns to teaching he must enrol in "an externally provided professional development course on maintaining boundaries", and must inform any new employer in the next three years about the tribunal's decision.

Coad sought name suppression to protect his children, who are now at school in Wānaka, but this was refused on the grounds that "there is no evidence provided that the effect on Dr Coad's children would be over and above the usual adverse effects that may occur to children where a parent faces disciplinary proceedings".

Coad told the Herald that he was "stupid" to have taken part in the Instagram chat, but that the tribunal had "misconstrued" some of his comments. For example, he said he posted a picture of his cats after the students asked him whether he was missing his children.

"I said, 'Yeah, I'm missing my cats.' I posted a picture of my cats. The students then started making reference to 'pussy'," he said.

He said he was under stress at the time.

"There were stresses in my life at the time related to my family, and frankly I was mentally not in a good place."

He said he offered to cancel his teaching registration "right there and then", and he was "really angry" that the Teaching Council decided that he still had to go through a full tribunal hearing which had taken more than a year to reach a conclusion.

"I struggle to understand where the public interest is, seeing as I have now left teaching and I told them they can have my teaching licence and I'm never going back to teaching.

"I don't know how anybody can endure the petty-mindedness of these bureaucrats and still be an effective teacher."

He said he could tell stories about other teachers that would "make their toes curl", yet the tribunal was "prepared to drag me through the mud".

"Frankly, they can go screw themselves and pencil-push some other poor sod to the point of breaking."