A fire at a recycling plant near Thames turned into an "ember storm" that was exacerbated by severe winds.
The winds turned the large fire into a complex and continually spreading inferno with airborne embers setting fire to a hay barn 500m away, Thames fire chief Greg Rendall said.
Firefighters worked tirelessly overnight to fight the blaze in Matatoki, near Thames, which is now contained, but still not out.
Firefighters continue to douse one of the biggest and complex fires the Thames area has seen in years.
Eastern Waikato area commander Aaron Waterreus said four trucks and about 20 firefighters from Rotorua, Hamilton and Tauranga continue to fight multiple blazes at the plant on Paeroa Kopu Road.
The fire erupted about 9.20pm on Tuesday at Smart Environmental's premises, engulfing piles of timber, wood chips and sawdust, along with plastic waste.
Strong winds blew hot embers across the site, resulting in several spot fires.
Thames fire chief Greg Rendall said the wind was so strong that a hay barn caught fire 500m away.
Rendall, who still hadn't managed to get any sleep by Wednesday afternoon after arriving at the scene shortly after it began on Tuesday night, said he was initially confronted with what could be described as "basically an ember storm".
"It was a large fire in a compost and timber pile and severe winds. The winds were really strong and so it was basically an ember storm. The embers were just blowing off across the complex and that was the issue, the wind and the embers.
"And consequently, with the wind, the fire grew quite rapidly ... to a fifth alarm. One of the highest alarms we've ever had in Thames."
With embers flying across the site, firefighters were continually coming across new spot fires.
"We had a hay barn that burnt probably 1/2km away. The winds were unbelievable. It was like the perfect storm, to be honest. It was a bit like watching one of those programmes in Australia, it was pretty amazing.
"We had some firefighters and commanders there that have been in the service a long time and they were pretty amazed by it.
"It was just one of those things, it just lined up."
While putting out the repeated spot fires, firefighters concentrated on protecting exposed buildings from the flames.
"There was probably only two buildings destroyed in it and the rest are still intact. Smart Environmental employ a lot of people in our town and without them there ... that impacts on our community in quite a big way for jobs and things."
Rendall said two aerial appliances remained at the scene today helping crews dampen down hot spots.
He said the volunteer crews worked through the night before being taken over by paid crews this morning.
He appreciated all the support from management and other brigades throughout the incident.
"We're pretty thankful for all the help and support. Everyone mucks in and gets things done."
Firefighters hoped to put the fire out but they knew it was going to be a big battle as they have to ensure the seat of every fire is fully extinguished.
Fortunately, conditions have improved vastly compared to last night, with gentle winds and even some sunshine helping the situation.
Waterreus labelled Tuesday's conditions "especially challenging" and said the volunteers were knackered after a big night.
"Our volunteers are absolutely shattered after a long night, and so have been swapped out with crews from other areas. We're so grateful for their efforts, working in such trying conditions all night and many of them have still had to go to their other jobs today."
The remaining firefighters are expected to continue their efforts through the rest of the day.
Waterreus said it was too soon to say what caused the blaze and where it began.