At least 17 people have sought medical attention after eating packaged fresh spinach that is believed to have been accidentally contaminated.
NSW Health said 47 people had so far reported symptoms after eating the green, leafy vegetable.
It is continuing to investigate following initial reports of at least nine people from four separate households across Sydney presenting to hospital.
People are being urged to throw out any packets of Riviera Farms-brand spinach, sold through retailer Costco, with an expiry date of December 16 this year.
NSW Health said the product was "not safe to consume and people who have it should throw it out".
It has urged anyone who has experienced any unusual and severe symptoms after eating the spinach to seek immediate medical attention by visiting their nearest emergency department due to toxic reactions.
State health authorities said some of the reported symptoms have been severe and include "hallucinations, delirium, rapid heartbeat and blurred vision".
NSW Health Secretary Susan Pearce said emergency doctors and nurses were able to identify the source of the contamination, after a number of young children presented to hospitals in Sydney with "severe illness".
"Staff within our emergency departments recognised and were able to pinpoint that quite quickly after the presentation of some children to our hospitals who'd been badly affected by it and very rapidly recognised that — joined the dots — and we got the alert out straight away," she said.
Ms Pearce said the children affected were in a stable condition, despite being unwell.
"I have had no further updates today to suggest that there's any deterioration there but delirium, confusion, hallucinations and so on, and just generally being very unwell."
Late on Friday, Victoria's health department issued a warning about the spinach, and said reports of unusual symptoms had been reported in both NSW and Victoria.
A Riviera Farms spokesperson said the product may have been "contaminated with a weed which can have health consequences if consumed".
"As soon as we were advised of the possible weed contamination from one of our customers, we immediately advised them to remove our impacted spinach from their shelves, and contacted state health and federal food authorities," the spokesperson said.
"There is no suggestion, and to our knowledge no possibility, that any other products have been impacted by this weed.
"Riviera Farms has been in the business of providing fresh food since the 1880s, and we are saddened to confirm that we have likely had our first-ever significant contamination incident."
NSW Health said it does not yet know the full extent of the product's distribution and is working with the NSW Food Authority, as well as other jurisdictions, to investigate the contamination further.
Riviera Farms' head office is based in Victoria.
President of the Victorian Farmers Federation, Emma Germano, spoke with the farm owners and said there was testing underway to identify the contamination.
"This is kind of the worst nightmare for a grower, to have some kind of food safety recall," she said.
"But it's very much a demonstration of how the food safety processes put in place work.
"They've gone back to the paddock that the batch that has created the issues was picked from and identified a weed that is in larger numbers than they expected."
Ms Germano said the suspected weed was a thorn apple.
