Police are trying to trace the movements of a boy found injured and distressed near Timaru.
The nine-year-old was found near the Holme Station Bridge on Thursday, about 15km from Ocean View Heights Primary School where he is a student.
He is in a stable condition at Timaru Hospital on Saturday. Officers are still waiting to interview the boy and will not do so until medical staff assure them it will not put him under any undue stress.
Detective Senior Sergeant Dylan Murray says police have spoken to several people who saw the boy before he left school, but still don't know what happened to him.
Mr Murray says the boy appears to have been assaulted and police are treating the incident as suspicious.
He says it is likely that the child was moved from the school to the bridge in a vehicle, but so far police have no description of what that vehicle might look like.
"At this stage, we're still not very clear on what's happened. So without wanting to scaremonger, it's probably a good idea for parents to continue with their normal picking up and dropping off of children that they do.
"Parents just need to use common sense and ensure that the plans they already have in place for the safety of their children are in place."
Police are asking the public to report any suspicious activity seen at the school on Thursday or sightings of vehicles around Holme Station Bridge.
About seven officers are working on the investigation.