Seventy cancer patients at Sydney's St Vincent's Hospital have been given incorrect doses of a chemotherapy drug for up to three years, an ABC 7.30 investigation has revealed.

Medical oncologist Dr John Grygiel had prescribed the same flat dosage of the drug carboplatin to patients with head and neck cancer without adjusting their individual test results and characteristics, according to the report aired on Thursday night.

All 70 patients were administered the incorrect dosages for up three years and in some cases were given as little as 50 per cent of the correct dosage, 7.30 reported.

"Obviously we're very, very sorry that this has happened," said Associate Professor Richard Gallagher, director of St Vincent Hospital's Head and Neck Service.