The 63-year-old on Tuesday pleaded guilty in the Bendigo Magistrates' Court to one charge of sexual assault.

The court heard the man picked up his step-granddaughter – aged in her early 20s – and her friend from from a central Victorian music festival after a storm prevented them from camping.

On the drive back to Melbourne late that evening, the victim offered the man $100 in payment for picking her and her friend up.

After some time the friend fell asleep in the back seat.

The man placed his hand on the upper thigh of his step-granddaughter, who was seated in the passenger seat, before saying “Why don’t I give you $100?”

The young woman was taken aback and asked what he meant, to which he replied it was to take off her top and “show me your boobs”.

The woman was shocked, and turned away from the man and pretended to sleep.

His hand stayed on her thigh for about one minute.

Several days later, the woman told her mother about the incident, and sent a text message to the man confronting him about what he did.

The man wrote two letters, one to his wife and one to the young woman, apologising for his actions.

In January this year, the man admitted to police he said he would give the woman money to show her breasts, but denied touching her.

The man’s defence lawyer, Rowan Skinner, said his client had committed “a spontaneous act of extreme stupidity”.

Mr Skinner said the man had given a “full and frank” interview to police, during which he said his actions followed an exchange between his step-granddaughter and her friend of videos of them partially naked at the festival.

The man’s behaviour was baffling to both himself and his wife, Mr Skinner said, and he previously enjoyed a very good relationship with his step-granddaughter.

The court heard he had been in her life since she was a young child.

In a victim impact statement read to the court, the young woman said the incident had greatly affected her life.

She said the offence had created deep divisions in her family: she had considered the man her grandfather and she was no longer in contact with her only grandmother.

The incident had also diminished her trust in men, she wrote.

The police prosecutor called for the man to be sentenced to a “short, sharp” term of imprisonment, followed by a community corrections order.

“It’s a breach of trust of the highest order,” he said.

But Mr Skinner submitted his client be sentenced with a fine.

He said the offending was at the lower end of the scale and his motivation for offending was not sexual satisfaction.

A report determined the man’s risk of re-offending was low, Mr Skinner said.

Magistrate Andrew Capell said he considered the man’s chances of re-offending were low, and imposed $7500 fine with conviction.

Mr Capell said the man’s actions were “at odds with anything (he’s) done before” and were “consistent with a curiosity in some respect”, but there was no “darker intent”.

He took into account the man’s guilty plea and letters of apology, and noted references from his son and employer.

But he said the man had committed a breach of trust against someone who considered him their grandfather.

The man had to live with a division in the family, Mr Capell said, that appeared irreparable.

“You’re the one that’s wholly at fault,” he said of the offence.

Mr Capell also granted a police application to collect a sample of the man’s DNA.

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