The now-75-year-old groomed his 16-year-old victim, leading her into sexual activity at his parents' home and taking advantage of her vulnerability, Judge Sarah Huggett found on Friday.
As Dawson watched on in silence by video link from Long Bay jail, the judge said he had a high level of moral culpability for the crime due to his role as a teacher.
"The offender did not use threatening words or conduct either to commit the offence or to secure the victim's consent and silence," she said.
"There was, however, a degree of manipulation and exploitation."
Dawson was sentenced at Sydney's Downing Centre District Court after being found guilty in June of one count of carnal knowledge as a teacher of a girl over 10 and under 17.
He plans to appeal the verdict.
The former teacher has shown a lack of remorse over his illegal sexual actions, which took place while he was married to the woman he later murdered, Lynette Dawson.
He pleaded not guilty at his trial, although he did not dispute he and the student had a sexual relationship.
Dawson's public-defence lawyer instead argued the sexual activity began when the student was no longer in the teacher's class.
On Friday, Judge Huggett rejected arguments by his counsel that the sentence should take place at the same time as that for his murder conviction, saying that the two crimes had different victims and occurred at different times.
In a report filed with the court, psychiatrist Olav Nielssen said Dawson had experienced cognitive decline while in prison, showing signs of the early stages of dementia.
The judge acknowledged that because of Dawson's age and mental state, he would experience a more onerous time behind bars.
She sentenced him to three years in prison with a non-parole period of two years.
However, the total sentence will be entirely subsumed by the 24-year jail period imposed for Lynette's murder.
The only practical effect will be that he will now be eligible for parole one year later, in August 2041, when he is 93.
The maximum jail term for the charge of unlawful carnal knowledge is eight years.
Ahead of the sentencing, crown prosecutor Emma Blizard said the teacher's offending was not a one-off but rather a course of conduct that led to his student being groomed over an extended period.
Dawson's lawyer Claire Wasley argued that the objective seriousness was lower because the victim was 16 years old and towards the maximum age of 17 set down by law for the offence.
She also said that an offence of this type by a teacher was less serious than similar conduct by a father or stepfather.
Last year, the NSW Supreme Court found the 75-year-old murdered his wife Lynette and disposing of her body in January 1982 to pursue an "unfettered relationship" with the teenage student.
While he was still married and living with Lynette, he groomed the girl and ultimately engaged in sexual activity with her before the end of the school year in December 1980.
The student said they first engaged in sexual activity at the Maroubra home of Dawson's parents, who were out of town.
Friday's sentence is ultimately unlikely to affect how long the 75-year-old spends in jail.
His full 24-year sentence for murdering Lynette Dawson, whose body has not been found after her 1982 disappearance, will expire in August 2046, after he turns 98.
Laws preventing parole before he discloses the location of her body mean he is expected to never be released.
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