Preliminary cost estimates for fighting a series of summer brushfires on O'ahu and the Big Island have reached nearly $500,000 and could grow as high as $700,000, officials said, making it the most expensive brushfire season in the state's modern history.

"I would say that this is the biggest, longest and costliest brushfire season we've had," said Eddie Chung-Hoon, who's in charge of coordinating federal fire grant applications for the State Civil Defense agency.

While his agency is still compiling the total cost to local, state and federal agencies, the Department of Land and Natural Resources alone has spent more than $300,000 on the season, while the National Guard has spent $145,000, primarily on helicopters.

"Normally the big costs are going to be the military helicopters and the DLNR," said Honolulu Fire Department spokesman Capt. Kenison Tejada. "Like at the Nanakuli fire, the military was there along with the DLNR, which doesn't have a helicopter and so it has to rent one from a private company."

Tejada said the fire department has spent a minimum of $12,000 to $14,000 in overtime pay alone so far this record year, and there are four months left to go.

When final estimates come in from a string of other agencies, it could push the total to nearly $700,000, officials said.

John Cummings III, plans and operations officer for O'ahu Civil Defense, said costs to his agency would not be substantial, considering its small staff.

"It's going to be overtime hours and costs involved in activating volunteers in the field," he said. "But the lion's share of the costs is going to come from the fire department."

Two of O'ahu's 700-plus brushfires this year have been so costly that for the first time the Island has applied for a Federal Emergency Management Agency fire grant that will pay for 75 percent of the expenses.

Based on cost estimates, Chung-Hoon said FEMA already has approved grants of at least $100,000 each for those two August fires — the second Nanakuli blaze that consumed nearly 3,000 acres, and a raging Kipapa Gulch fire that burned 200 acres and sent smoke into the Royal Kunia, Village Park and Waikele neighborhoods.

"Based on the estimates, we feel we have already met the $100,000 threshold for each of those fires," he said. "We haven't really looked at the actual costs."

He said State Civil Defense is collecting supporting data from every agency involved and will tally up the expenses.

"They have to be substantiated," he said. "You can't just throw out numbers. You have to present the data that supports the figures."

And while Chung-Hoon is not positive the Waikele fire will reach FEMA's required $100,000 threshold, he's reasonably sure it will. But he said he has no doubt that the Nanakuli fire will surpass the threshold.

"That's a for sure," he said. "That's where the cost is going to run up."

Still to be collected are figures from other federal, state and county agencies involved, including O'ahu Civil Defense, State Civil Defense, the Army, Marine Corps, Navy and FedFire.

Getting all the pertinent data together is time consuming, said Tejada of the HFD.

"Right now we're compiling figures for the year, and for just those two fires," he said. "But, it's not ready yet."

More than half of the brushfires this year have happened in west O'ahu, and investigators believe the majority have been deliberately set.

For now, Chung-Hoon said it's not possible to guess this season's total brushfire tab. He only knows it will be high.

"To be honest we really don't have the figures yet," he said. "Next week, I'm calling a huddle with the county guys to start talking about the collection process. I've got to brief them on the process on how to submit the costs.

"Because we've got to stand an audit test."

Reach Will Hoover at whoover@honoluluadvertiser.com.