An elderly man who committed a string of indecent assaults against former Greens leader Bob Brown in the 1950s has been jailed for sex offences relating to a Sydney boy in the 1960s.

Ian Douglas Davidson, 84, will be behind bars for at least three years after he was found guilty of eight counts of indecent assault and one of buggery committed against an 11-year-old while Davidson was a teacher at Sydney Grammar School, Sydney District Court heard on Friday.

During sentencing submissions, crown prosecutor Margaret Cunneen SC drew on tendency evidence given earlier in the case by Mr Brown.

Davidson was charged and convicted of six counts of indecent assault against Mr Brown, then a schoolboy, in Armidale, NSW in 1957, court documents state.

Then a young teacher, Davidson failed to declare his convictions to Sydney Grammar School when it employed him, the documents say.

They also state that during the period of abuse, the victim was scared he would be in trouble if he refused Davidson’s advances.

In one particular incident, the victim screamed out in pain, Judge Deborah Sweeney told the court.

‘It was clear from (the victim’s) evidence that he was acutely conscious of other boys becoming aware of his abuse,’ she said.

It wasn’t until about 50 years later that Davidson was arrested at his nursing home in the inner Sydney suburb of Surry Hills.

When questioned by police, Davidson denied knowing or teaching the victim during his time at Sydney Grammar, despite police showing him copies of the victim’s report cards which he had signed, the court documents say.

Judge Sweeney said the victim had told the court that the abuse left him struggling with ‘anger, fear, loss of trust, self loathing’, poor relationships with family, self harm and a dependency on alcohol and drugs.

In giving her considerations for the length of sentence, Judge Sweeney said she took into account that Davidson suffered from a range of medical conditions including heart problems, high blood pressure and hepatitis B, that he was very old and did not have a long life ahead of him. Davidson has been sentenced to seven years in jail and will be eligible for parole after three years, in April 2019.