The Bendigo man, 85, was convicted in Bendigo County Court on Friday on three counts each of incest and committing an indecent act with a child under 16.

He had pleaded guilty to the charges.

The court heard the man molested one girl five times during a two-year period.

She was 9 when the abuse began.

He would confront the girl alone at family gatherings, including Christmases, and indecently assault her.

After the first incident, he told her not to talk about what happened, telling her it was "our little secret".

On one occasion, he forced her to perform a sex act on him at his partner's birthday party.

He was also convicted of molesting the girl's younger sister, also 9 at the time of the offending.

The abuse occurred about a decade ago, when the man was in his mid-70s.

The sisters were the granddaughters of the man's partner of 25 years.

Victim impact statements from the girls' parents described a devastated family that had worked hard to get through the pain.

"The devastation of the news left us in complete shock," the girls' mother wrote.

The sisters stated they had difficulty forming relationships with men and had felt angry about the abuse, but were now looking forward to a more positive future.

During sentencing, Judge Paul Lacava said the offending had been opportunistic and was unlikely to reoccur given he had not re-offended in the past decade.

He said the man had no prior convictions, and his guilty plea indicated genuine remorse for his actions.

Judge Lacava also noted the man would find prison more difficult because of his age, and was likely to die before his release.

But, he did not accept the defence's submission a wholly suspended sentence would be appropriate, saying the three-year maximum penalty available to suspended sentences would not be enough.

Judge Lacava sentenced the man to five years in jail with a non-parole period of three years.

He said had the man not pleaded guilty, he would have jailed him for seven-and-a-half years, with a minimum of five years to serve. The maximum penalty for incest is 25 years.