Police were called soon after.
Hamilton Police Detective Constable Marcus Hunter confirmed police were investigating the incident, and had spoken with the boy, who was uninjured, and his mother.
The man was reported to be Caucasian, in his early 20s, with black hair, wearing a black T-shirt with a white skull on it, Hunter said.
The Facebook post has been shared dozens of times and has drawn concerns from some parents.
One mother wrote that unless the safety of her son could be guaranteed, "I am seriously considering not sending him at all".
"My son walks home after school we are close to the lake. I am terrified for his safety.
"If there are any like minded parents who would like to discuss strategies for our kids who for all other purposes are left to walk home I would be very happy to hear it and contribute."
Another asked if there were any further safety measures that could be taken around pick-up and drop-off time.
"One adult staff member at each crossing watching 100s of children cross a road safely is not nearly enough."
The school posted another message to parents, advising it was aware of the concerns and adding that the school last term taught the Keeping Ourselves Safe Programme to all students and that this would continue in the future.
"This is now a Police matter and if required they will advise us of any further actions."
School principal Kirsten Ratana has not yet responded to requests for comment.
Police were appealing for anyone who witnessed the incident to phone Hamilton police on (07) 858 6200.