It was believed his father, farmer Simon Mackle, was moving some cattle and standing at a gate when the vehicle Alex was in started rolling backwards.
As Alex tried to escape he fell underneath the vehicle.
On Friday, emergency services were called to the property on Inland Kaikoura Rd at 6pm.
A police spokesperson said the matter was being investigated.
WorkSafe would also be investigating the incident.
According to WorkSafe statistics six people died working in the agricultural sector from vehicle related injuries last year.
Four of these incidents involved quad bikes.