The spokesperson said the spill was poultry oil.
Tauranga Senior Sergeant Rob Glencross said the total amount spilled on SH29 was not known but "it was a considerable mess".
"Particularly on our side of the summit but it also extended down the other side as well."
"I don't believe the driver knew what had happened.
"As far as I know, it was a fault in the tank that caused the stuff to be released.
"It was a fault in the operation of the vehicle."
Drivers were forced to travel on alternative routes, which were through Waihi and Katikati, or through Pyes Pa or along the State Highway through Te Puke to Rotorua.
"The traffic team are investigating the incident and will determine whether the driver will be issued with an infringement notice for unsafe operation of the vehicle as a result of the discharge of material," Mr Glencross said.
"The driver of the vehicle needs to make sure his load is secure and failure to do so is an offence. So he could be liable, yes."
The public alerted police, who closed the road between Poripori Rd on the Tauranga side and Raparapa Rd on the Matamata side, at 10.25am.
Traffic had spread the tallow by the time police intervened.
Waikato Regional Council's ready response team went to the site and spread pumice to absorb the oil and detergent to remove it.
Streams were assessed but had not been affected.
SH29 was reopened at 4.20pm.
New Zealand Transport Agency state highway manager Kaye Clark said the clean up operation was to continue today and crews would remove the last of the sand put down.
That section of State Highway 29 carries 8000 to 9000 vehicles each day.
Tallow is imported and exported through the Port of Tauranga. Port staff said the load could have been coming in, however staff weren't sure.
Bulk Storage Terminals on Totara St is one of three sites within the port that stores tallow.
Terminal manager Grant Van Oudenaaren said tallow was a non-hazardous material and stored in bulk tanks.
"It basically comes from rendering animal carcasses, it's a product of the rendering process," he said.
"It's shipped from here to overseas locations, generally for industrial use."
Tallow is solid at room temperature but heated and turns to liquid when it is being transported.
"The state of tallow is a liquid. It's lowered into a tank at 60C in a liquid state but it's like lard.
"It contains a lot of fat.
"When it's heated it's mobile and when it cools it turns into something similar to what lard does - essentially it is lard," Mr Van Oudenaaren said.
"It's very oily and slippery. I can imagine the damage done to the road."
Bulk Storage Terminals was not expecting a truck of tallow on Saturday.
The other sites that store tallow are Marsels Terminals and Pacific Terminals.
Tauranga's Sharon Archer commented on the Bay of Plenty Times' Facebook page about the spill: "We went from Tokoroa to Rotorua to Tauranga. Took us a wee bit longer but with the rain it was probably a blessing as the top of the hill would have been wet and foggy," she wrote.
Police wanted to thank the public for their patience on Saturday.
"It did cause considerable inconvenience to people who wanted to use the road. It was closed for quite some time and the police are grateful for the patience of people," Mr Glencross said.
