A Sydney dentist has been found guilty of filing down the teeth of nursing home residents without their consent.

Andrew Istephan was accused of carrying out unauthorised surgery on a dozen nursing home residents in Lewisham, Leichhardt, Haberfield and Manly Vale.

A District Court jury has found him guilty of five counts of assault after a week of deliberations but has been unable to reach a decision on another seven counts.

He is likely to stand trial for those again at a later date.

During the trial, the jury heard only three patients were mentally fit to give their consent for the work, which involved the filing of teeth in preparation for crowns.

Some patients were aged in their late 90s and some had dementia.

Istephan said he thought the nursing homes had obtained patient consent and consulted with their families.

The court heard the work was carried out under the Federal Government's Chronic Disease Dental Scheme, which no longer operates.

Under the scheme, patients whose oral health was considered likely to affect their general health could be referred to a dentist by a GP and have the surgery paid for by the government.

Istephan is also a councillor on Hurstville Council.

The 34-year-old has been ordered to surrender his passport until his sentencing hearing later this month.