Ontario's watchdog says Hydro One's billing practices have garnered an unprecedented number of complaints to his office.

Ombudsman Andre Marin says so far, he's received 7,900 complaints about the provincial utility — and the number keeps going up. His annual report says it's the most complaints his office has ever received about a single government organization.

Marin launched an investigation in February after customers complained about delayed or missing invoices, which resulted in high "catch-up" bills.

His report says the Crown corporation took $8,390 from the bank account of one woman who had authorized automatic payments, saying they'd underestimated her billings for almost two years.

Marin says Hydro One responded immediately after he announced the probe and have taken several short-term measures to alleviate problems.