The crash happened when a seven-seater people-mover with a mother and five children inside collided with a car with a mother and baby on board about 3pm.
Police and Fire and Emergency New Zealand crews responded to the crash, which occurred at the intersection of Itchen and Severn Sts.
As there was no ambulance available, police took the mothers and children to Oamaru Hospital to be checked over.
This evening, Hato Hone St John Rural Otago area operations manager David Milne said its Oamaru ambulance was at a higher-priority incident at the time, and it responded to the crash after that but was "stood down by another emergency service as an ambulance was no longer required".
Earlier, Sergeant Tony Woodbridge, of Oamaru, said he was unsure about the seriousness of any injuries.
"Hopefully these wee kiddies aren't too seriously injured. They got a big fright, obviously, and it's just a matter of them being looked after here at the hospital."
When asked about the hospital transport, he said ambulances "must have been off at other incidents".
"I'm unsure what the story is with that.
Police are investigating.