A university professor convicted for the indecent assault of an 82-year-old rest home resident was investigated over a second sexual assault complaint, also involving an elderly woman.

The Herald has learned police sought to charge former Massey University journalism lecturer Grant Hannis with a second crime but were unable to gather enough evidence to prosecute.

Hannis, who was sentenced to eight months' home detention in January after pleading guilty, denies there was a second assault and said he was unaware of any other investigation.

The 55-year-old, once a celebrated Wellington academic, argued during sentencing that his attack on the dementia sufferer was "not lengthy", there was "limited premeditation" and it should be seen as "opportunistic offending".

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However the Herald understands aspects of the second complaint were similar to the first case. The alleged victim was also in a rest home, and although not in a dementia ward, she was also deemed by police as extremely vulnerable.

Further details of the second allegation have not yet been released due to the victim's wish for privacy.

Police said the second complaint did not proceed to prosecution because it did not meet the evidential threshold. For a case to go to court in New Zealand there needs to be enough credible evidence for there to a "reasonable prospect" a jury will convict.

Around 80 per cent of aggravated sexual assault cases where police believe the victim are not prosecuted, mainly because they do not meet that threshold.

Detectives began investigating the second allegation after Hannis was caught with the original victim in her bedroom at a rest home in the North Island in May last year.

Hannis was visiting a relative at the rest home, which he did regularly. He also frequently visited other retirement villages as part of a jazz band.

Court documents said Hannis first struck up a conversation with the victim, then followed her back to her bedroom, where he assaulted her.

The woman, who had a stroke in 2014 and whose condition has deteriorated since the attack, told police she tried to push him off but he was "too forceful" and "wouldn't stop".

Hannis was caught only because a caregiver opened the door and witnessed the assault.

During sentencing, the woman's daughter described how after the attack, her mother hasn't been the same, the "bright spark" in her eyes now dull. "Our mum has slowly drifted away."

Initially, Hannis denied sexual contact with the victim. He then suggested she'd "done it to herself". Later, he said the interaction was consensual, and that the pair were "private lovers".

He refused to plead to a charge of unlawful sexual connection, only admitting guilt when it was downgraded to indecent assault as part of a plea bargain - agreed to by police in part to save the elderly victim the trauma of a trial.

Hannis tried hard to keep his name a secret, arguing he should get suppression because he was mentally unwell. His request was denied.

During the sentencing process, Judge Stephen Harrop earlier described what Hannis did as "unbelievable offending".

When calculating the sentence, Judge Harrop took into account Hannis' previous good character, his contribution to the community, and his remorse, as well as the decision to lift name suppression.

Harrop sentenced Hannis to eight months' home detention, 100 hours' community work and ordered him to pay $3000 in emotional harm reparation.

Hannis continued teaching for six months after his arrest. Massey University said it was never told about his arrest or the charges he was facing.