When they went back to check, a tunnel he was building had collapsed on him, his feet and ankles were hanging out, and his head was right down at a depth of about 50cm.
Thomas said he was pulled out, sand around his mouth was cleared before CPR was performed on him as a St John ambulance was 10 minutes away.
The school bus driver and parents helped keep everyone calm.
"It was pure accident but what was great was that the CPR worked. The principal and another teacher went through the processes that had to be followed like checking signs of alertness, making sure the airway wasn't blocked," Thomas said.
"These are great rural kids and they really care about each other. They're just little ones, there was no meanness or anything like that intended when they ran off to their school bus, leaving him behind."
Thomas went to the school and the principal spoke to her about the incident.
"Our principal will walk on water for her students. There's such a surge in adrenalin when things like what happened happens, you've got someone's life in your hands but she did well and her actions saved the child."
The boy was admitted to the Whangārei Hospital for a day or two and made a full recovery.
One the school board of trustees became aware of the accident, Thomas said they shut the sand pit, took immediate advice from relevant authorities, and carried out a full audit to ensure the sandpit was as safe as possible.
She said the school policy was that all staff must hold a current first aid certificate and that it be upgraded regularly.
St John continues to advocate for CPR and lifesaving first aid skills to be taught and included in the national school curriculum, in line with the government's "School Leavers' Toolkit" policy that every school leaver will graduate with basic first aid education.
"We are totally supportive of the call from St John for everybody to learn first aid and in particular CPR. We know how important it is, for as many as possible to have these skills and knowledge, because you never know when you will need them," Thomas said.
"I can't help commending our principal Megan Tebbutt for her reaction. She knew what to do. She reacted without panic, and putting it bluntly, she saved the child."