The herbicide glyphosate had been sprayed in liberal doses, with the weed killer’s stains clearly visible around the playground the next day, even after rain.

It was dyed blue so workers could avoid double spraying. The residue could be seen over many heavily-used areas of the playground, where toddlers frequently play.

Wollongong City Council says the herbicide is safe and is approved by Australian authorities. 

But Bulli mum and horticulture student Heather Carey said the poison was sprayed in reckless quantities, when there are other, safer ways of removing weeds.

“They just sprayed willy-nilly,” she said.

“You could see it on the chair. It also goes into creeks, into groundwater. I thought it was quite attractive to little kids – nice bright blue foliage to pick up.”

Ms Carey said she was concerned no signs had warned park users of the spraying – but a council spokesman said the signs were put up as required when the spraying was taking place.

“Council provides notification to the public as per its pesticide notification policy when it carries out routine weed suppression,” he said.

“Council adheres to guidelines issued by the Australian Pesticides and Veterinary Medicines Authority which provides advice on the safe use of pesticides. 

“Using a pesticide is an effective method to remove a broad range of leafy weeds from specific areas.”

Glyphosate – sold under the brand name RoundUp, and produced by global chemical giant Monsanto – is perhaps the most widely used herbicide in the world, and Monsanto maintains it is safe for use around humans.

But anyone using it is advised to wear gloves. 

Glyphosate’s use is restricted or banned in some countries, while the World Health Organisation’s cancer agency has concluded it is “probably” a carcinogen.

There are also many court cases in the pipeline over glyphosate, mostly in the US.

Last year a California court ordered Monsanto to pay more than $406 million (later reduced to $100 million) to a former school groundskeeper who was dying from non-Hodgkins lymphoma, after prolonged exposure to RoundUp in his job.