A digger will dismantle the charred and smouldering remains of a South Island mill as firefighters dampen flare-ups nearly 12 hours after the blaze started.

The spectacular fire, which broke out at animal feed manufacturer SealesWinslow near Ashburton about 7.40pm, has destroyed a giant processing shed.

Three firefighters were taken to hospital with burns.

Fire and Emergency southern spokesman Stephen Butler told the Herald smoke was still rising from the smouldering ruins.

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A heavy digger would this morning demolish parts of the blackened structure to enable firefighters to reach hot spots.

Fire tears through the Seales Winslow animal feed plant near Ashburton yesterday evening. Photo / Ashurton Guardian
Fire tears through the Seales Winslow animal feed plant near Ashburton yesterday evening. Photo / Ashurton Guardian

"We're about to start up operations again to get the fire out and fully dismantle the building," said Butler.

"We had a digger do some work earlier and there's more to do."

Nearly a dozen firefighters remained at the Ashburton property overnight.

Butler said one of the injured firefighters had already been discharged from hospital after the trio suffered minor burns.

Meanwhile, North Island fire crews spent the night battling a large industrial blaze at a wood processing plant in Tokoroa.

The blaze broke out at the large plant on Balmoral Dr just before midnight.

Nearly three dozen firefighters from as far afield as Rotorua fought four four hours to bring the fierce blaze under control.

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Fire and Emergency northern communications shift manager Paul Radden said a passing train driver saw flames coming from the building and raised the alarm.

Five crews are damping down hotspots before specialist fire investigators start working out how the fire started.