Two men working across the road at the Firestone tyre store also saw the smoke and ran across the road.
They and Mr Ngarimu climbed into the back of the vehicle and pulled out the unconscious children.
Mr Mutu revived the boy while another man resuscitated the three-year-old girl.
The two youngsters suffered "serious burns'', Fire Service central communications shift manager Belinda Beets said.
"They were heroes, those guys,'' said one witness.
St John paramedic Jackie Clapperton said staff from nearby businesses did a good job getting the children out, reviving them and treating their burns.
St John district operations manager Stephen Smith praised the work of the witnesses who rescued the children, and said they may have made the difference in saving the children's lives.
"From our point of view, removing the children from the fire and providing that initial first aid has contributed greatly to their care.''
"I just hope the kids pull through,'' Mr Ngarimu told the Gisborne Herald.
Mr Smith said members of the public who pulled the children from the burning car performed first aid with the guidance over the phone by a St John paramedic.
Two ambulances with specialist paramedics arrived a short time later, and the children, one unconscious, were rushed to Gisborne Hospital.
"I believe they will be evacuated out of there, probably to Auckland.''
He said the children's mother was believed to have been ``in the vicinity'' at the time.
The children were believed to be siblings, aged two and three, Mr Smith said.
They were in a critical condition with burns to about 40 per cent or more of their bodies.
An air ambulance was on the ground early this evening, ready to evacuate the children for specialist care.
Fire safety officials and police are investigating the case of the blaze.
