Both children were removed from the car and treated by ambulance staff at the scene.
At the time MetService recorded outside temperatures of 29 degrees.
Specialist weather data was being collected by police, Fairfax reported.
The police said Metservice data would help pinpoint the temperature inside the gold-coloured Toyota at the time.
Detective Sergeant Terri Wilson, of Hamilton police Child Protection Team, said the car had been parked in the space for 12 minutes before the first bystanders arrived.
"We're waiting on the data to get an accurate estimation of what the temperatures were on the day at the Base, and through their experts work out how hot it would have been inside the car.
"It's a serious incident but there is a lot of different experts that need to be involved in it," Wilson told Fairfax.
Information on the temperature inside the vehicle helps to formulate any potential charges, he said.
Police were also investigating whether the same grandfather also left two 11-year-olds and a 10-year-old unsupervised at a local pool complex, about 10 minutes drive from The Base, while he went to get lunch.
Police had spoken to the parents of all of the children, who came from the same family living in Hamilton.
"They were naturally upset about what had happened," Wilson told Fairfax.
Various witnesses have been spoken to and Police are calling for a woman who gave first aid by applying wet towels and ice to one of the children rescued from the vehicle.
Detectives were in the process of obtaining and reviewing CCTV footage from the Base carpark area where the children were located.
Police would like to hear from any further witnesses who may have seen the children inside the gold-coloured Toyota vehicle on the afternoon of January 28.
Charges have not been laid yet, however this will be considered by mid-week.
