Health authorities in Australia have said nine people in Sydney suffered "toxic reactions" -- including hallucinations -- from a batch of spinach thought to contain an "accidental contaminant".

New South Wales Health said people from four different households needed medical attention following "possible food-related toxic reactions" caused by tainted Riviera Farms spinach bought at wholesale giant Costco.

Health officials say initial investigations suggest the reactions were caused by the presence of "an accidental contaminant in the food product".

The health authority warned of "severe" possible symptoms associated with consuming the spinach, including delirium or confusion, hallucinations, rapid heartbeat and blurred vision.

Officials say the baby spinach bought at Costco with an expiration date of December 16, "is not safe to consume and people who have it should throw it out".

NSW Poisons Information Centre medical director Dr Darren Roberts was quoted by the Sydney Morning Herald as saying that some patients were still ill more than a day after the onset of symptoms.

"The patients that have been quite unwell have been to the point of marked hallucinations where they are seeing things that aren't there," he said. "They can’t give a good recount of what happened.

"No one has died, so we're very happy with that and we hope it remains that way, but these people are quite sick."