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Fire investigator Sam Bugley said it is no longer in his hands and police will be following up on "some leads".
Twenty-five firefighters battled the massive fire that pumped toxic black smoke over parts of Greymouth on Tuesday night.
Residents 3km away reported hearing the "whoosh" as the fire first flared in the used tyre pile near the port, behind Aratuna Freighters' fertiliser warehouse.
The spectacular blaze drew a crowd, with spectators lining either side of the Blaketown Lagoon.
Greymouth Volunteer Fire Brigade chief fire officer Lee Swinburn said the intense fire, fed by the stack of 800 to 1200 truck tyres, was thought to be suspicious.
Five appliances from the Greymouth and Cobden brigades responded to the alarm at 10.15pm; most were stood down at 3am when the fire was under control, while one remained on site until 8am.
Swinburn said Aratuna Freighters staff moved truck, trailer and storage units away from the fire, which did not damage any infrastructure or machinery.
"It was good to get it out before daylight."
Fire and Emergency NZ asked residents south of Blaketown to close their windows, remain indoors and stay out of any remaining smoke where possible after a northeasterly breeze fanned the acrid smoke over southern parts of the suburb.
Aratuna Freighters general manager Andrew Havill confirmed there was no damage to the company's infrastructure, "thanks to the fire brigade."
A "pile of rubbish" was all that was left of the tyre stack.
"We will wait to see what the authorities have got to say about what we can do and get on with it."
Havill said the storage site was difficult to access "but that never stops a criminal."
Fire investigator Sam Bugler said the investigation is now in the hands of police.
He could not provide any details but said police will be following up on some leads.