A car workshop fire that sent ash flying onto neighbouring properties is thought to be suspicious.

Piles of burning tyres helped fuel the massive blaze at Speedy's car yard in Kohunui St, Ngatea, Tuesday night.

Waikato fire safety investigator Ross Fleet confirmed the fire is being treated as suspicious.

Northern fire communications shift manager Jaron Phillips said the workshop at the car yard was well alight when fire crews arrived just before 9.30pm on Tuesday.

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Seven fire crews from Ngatea, Turua, Thames and Hamilton helped bring it under control.

Phillips said residents from as far as two blocks away reported ash falling on their property.

Piles of tyres burning inside and outside the building helped feed the fire, Fleet said.

Fleet, along with police CIB and forensic staff, were at the yard this morning.

He estimated the building to be about 300sq m and big enough to house five vehicles.

However, only one vehicle was in the shed at the time.

He believed the shed was used to store impounded vehicles.

It was unclear where in the building the fire began.