Following an intensive investigation, detectives from the Northern Investigation Section advised two teenagers - aged 13 and 14 - were arrested on Monday afternoon.

"They were both charged with arson," Detective Senior Sergeant David Richardson said.

"The two males were remanded in custody and will appear by via video link in Darwin Youth Justice Court today.”

EARLIER: July 6, 9.30am

POLICE are investigating a suspicious fire that has caused about $100,000 damage to a building at Katherine High School overnight.

Detective Senior Sergeant David Richardson said police and Northern Territory Fire and Rescue Service crews received a report that a building at the rear of the Katherine East school was on fire at about 6.30pm on July 5.

“It appears the fire started in a storage area at the rear of the building, which then spread to the rest of the main building,” he said.

“Fire crews extinguished the flames at around 9pm, with the building sustaining extensive damage.

“Police estimate the damage bill to be well in excess of $100,000.

“Fortunately, no one was injured.”

He added that there was a report of a “large group of youths in the area prior to the fire”.

A crime scene has been established and detectives from the Northern Investigation Section are investigating the blaze.

Katherine fire station officer-in-charge Chris Manuel said it took nine firefighters more than half an hour to extinguish the fire.

“We had all three fire appliances in attendance,” he told the Katherine Times.

“About nine fire officers brought the fire under control in about 30 minutes.

“We were actually able to save about 75 per cent of the building, which is a good result.”

Police are urging anyone who may have witnesses suspicious activity in the area between 5.30pm and 6.30pm on Sunday to call 13 14 44 or Crime Stoppers on 1800 333 000.