The man is being sought by police after the attempted abduction from outside St Matthews Primary School in King St shortly before 9am.
The man began following the siblings as they were walking to school together, police said.
The little boy was following behind his sister, and as they reached the school entrance, the man grabbed the boy by the hand and started leading him down the road, police said.
The youngster cried out, and his sister turned around and called out to him.
The man then let go of the boy's hand and fled the scene, police said.
The girl immediately ran to the school and told an adult, who phoned police.
Police this afternoon released photographs of a man who was seen in the area at the time of the incident.
"We are very keen to speak to this person and we need to clarify if he was involved," Detective Rob Parker said.
"This has been particularly upsetting for the children and we need to ascertain what, if any, involvement this man had."
The attempted abduction was also "a timely reminder to parents to remind their children about 'stranger danger"', Mr Parker said.
"Unfortunately we do have these types of incidents occurring every now and then, so it's incumbent upon parents to educate their children about not going off with strangers and what to do if they feel they are in danger.
"It's important that children know to raise the alarm in these circumstances," Mr Parker said.
Anyone with information about the man's identity or anyone who may have seen the incident is asked to contact Mr Parker on 06 831 0700.
