A teacher has been deregistered after marrying a girl he taught at primary school who is 34 years younger than him.

The Teachers Disciplinary Tribunal has found that the teacher's behaviour over a period of years was "at the most extreme end" of serious misconduct and the only appropriate penalty was cancelling his teaching registration.

His name has been suppressed because he is now married to his former student and she has adopted his surname, so publishing his name would also identify her.

Both the teacher and his former student gave evidence that their relationship began after the former student turned 16, and that she was 20 when they married a few years ago.

The tribunal found that the former student began visiting the teacher in his role as her former primary school teacher, and not as an independent adult.

It highlighted the 34-year age gap between them and concluded that the teacher "embarked on a deliberate and consistent pattern of behaviour in pursuit of a vulnerable young woman".

The teacher taught the girl who is now his wife when she was in Years 7 and 8, when she was aged 11 to 13, at a primary school in what was described as a rural "village" in the North Island.

The girl's cousin, who was four years younger, gave evidence that the older girl - called "Student C" in the tribunal decision - sometimes stayed behind after class talking to the teacher, called the "respondent".

"Her evidence is that she saw their hands touching a few times and when she walked in there would be an awkward silence and they would look at her and shuffle apart," the tribunal said.

"She described the conversations she overheard between the respondent and Student C as "not normal". She heard them talking about what Student C wanted to do when she was older. When she walked in, the respondent would sometimes say, 'Student C, you can go now.'"

The young cousin said that at the end of the older girls' Year 8 year the teacher gave the older girl money and jewellery and also gave the young cousin $5 and some earrings.

However the teacher's lawyer pointed to inconsistencies between the young cousin's initial statement to investigators and what she later told the tribunal, and the tribunal found there was insufficient evidence to prove that the teacher engaged in any serious misconduct while his future wife was at his school.

The teacher told the tribunal that he immigrated to New Zealand with his wife but she later died. He said he had been in other relationships since her death.

There was disputed evidence about when the former student started visiting the teacher at her old primary school.

The primary school principal gave evidence that the former student and a male friend, also a former student of the school, continued to visit their former teacher well beyond the ages when other former students commonly visited their old teachers.

He said the former student who later married the teacher was "quiet and withdrawn, lacking in confidence with a limited number of friends". She needed in-class support.

The principal talked to the teacher about the need to observe "professional boundaries" when the former students visited.

The tribunal found that the sole reason the two former students visited the teacher was that he had been their teacher and "they thought he was a good teacher". The former student addressed him as "Mister" for some years.

The former student said she usually took a 10-to-15-minute bus ride home from college, did chores and then she and her friend visited the teacher.

"At a best guess this would have been close to 4pm at the earliest, many staff would have left for the day by this point leaving the students and the respondent that much more vulnerable. This should have raised alarm bells very quickly for the respondent," the tribunal said.

The former student and her friend said they went to a movie with the teacher.

Her friend said they visited the teacher's house "at the beach" and "hang out".

Three photos taken showed the teacher with his arms around his former student and in one case with his hand on the student's inner thigh.

The former student's parents gave evidence that they found out about the relationship six months after their daughter had left college. She was then aged 18.

"Student C had told her parents that she was in [a North Island town] that weekend with three of her friends, but she had actually spent the weekend with the respondent," the tribunal said.

The former student agreed to meet her parents at a cafe in "the town" to discuss the relationship. She agreed to travel overseas, where her brother lived. Her parents "set her up with some money and a car".

However, the teacher followed her overseas and she returned to New Zealand. She stayed with her parents for about a week and then moved in with the teacher.

Her father said he went to the teacher's house a month later, took his daughter out for a walk and tried to drive away with her. He said the teacher tried to stop him and there was "a physical altercation" between them.

The father took his daughter to stay at a friend's house, but three days later she jumped out the window and hitch-hiked back.

The teacher and his former student eventually married. The former student described her wedding day and marriage as "the happiest time of her life".

"[The teacher] means the world to me. He treats me with total respect; he supports me in everything that I want to do and comforts me when I'm feeling low," she said.

But the tribunal found that the relationship was inherently unequal and that the former student was "totally dependent" on the teacher financially, socially and emotionally.

"Her role in the relationship is about fulfilling his needs and wants. The power imbalance now that they are married is as significant as it has always been," it said.

"His behaviour is about power and control not partnership and reciprocity."