(CNN) — An Australian physical education teacher who murdered his wife so he could start a new life with a teenage student has been convicted of sexual assault.

Sydney Downing Center District Court Judge Sarah Huggett sentenced Chris Dawson on Wednesday for “carnal access to a girl over the age of 10 and under the age of 17”.

The charge concerns the same student with whom Dawson started a new life after murdering his wife Lynette in 1982. He later married the student and had a child with her, but they later divorced.

Dawson was convicted last year of murdering his first wife, Lynette, in a conviction that ended one of Australia’s longest-running cases.

He had long denied the murder, saying Lynette left him and their two young children behind before disappearing.

The student, who can only be identified as AB, alleged that Dawson sexually assaulted her in 1980, when she was a 16-year-old student of his, during one of his classes at Cromer High School in Sidney.

Dawson, who appeared in court via video link from Long Bay Jail in Sydney, denied this, saying he started the relationship with AB when she was 17 and no longer his student.

Judge Huggett sided with AB and found that Dawson had formed an emotional connection with the teenager in order to abuse her while she was still his student.

AB said she started babysitting Dawson when she was 16, often spending nights at Dawson’s house.

“He asked me to marry him when I was 16, many, many times, I always felt compelled,” AB alleged.

Judge Huggett said she accepted evidence that Dawson proposed to AB at least once in 1980, when AB was 16.

The judge found there was “powerful evidence” in a 17th birthday card sent by Dawson to AB in early 1981, in which “the defendant, a mature man, as opposed to an immature teenager, relied on the existence of a reciprocal and permanent relationship. And that was because he had started a sexual relationship” in 1980.

Dawson dedicated the card to “the most beautiful girl in the world” and wrote “knowing that we will share all the birthdays that follow”.

The body of Dawson’s wife was never found. He is not eligible for parole while the body is still missing.

AB had testified against Dawson in the murder trial, during which Dawson was jailed for 24 years. The judge in that case found that the allegations that Dawson sexually abused his 16-year-old student in 1980 were “true and reliable” and that Dawson’s obsession with the teenager was enough to motivate him to kill Lynette.

Dawson responded to Wednesday’s verdict by repeatedly swearing.

He will be sentenced for “carnal access” on September 15 this year.