Placing him in the child seat of the trolley, Mikayla noticed her son becoming quite restless.
But it was only when she lifted him up that she realised his leg was bleeding from a compass needle had been wedged into the frame of the trolley.
Speaking to 7News Mikayla said she hoped the needle wasn't deliberately planted but admitted she wasn't prepared to take the risk of placing her son in a child trolley seat again.
'I had a mum freak out... it's an evil act,' she said, revealing she had checked the seat before placing her son in it but she hadn't noticed the compass.
Mikayla also added that Kai had since had a blood test to rule out any blood-borne diseases.
A spokesperson for Big W said the trolleys were cleaned and checked regularly by staff throughout the day.
'The safety of the families that shop in store it its absolute priority'.