A firefighter and his brother have been jailed over four deliberately-lit bushfires that swept through parts of the New South Wales Hunter region in 2017.

Key points: A firefighter who encouraged his younger brother to light bushfires in NSW has been jailed for five years

A firefighter who encouraged his younger brother to light bushfires in NSW has been jailed for five years His brother received a lesser sentence after admitting his guilt early and cooperating with police

His brother received a lesser sentence after admitting his guilt early and cooperating with police The fires threatened homes, a hospital, and an aquatic centre in the Hunter Valley, leading to mass evacuations

Retained Fire and Rescue worker Joshua Lambkin, 31, was handed a five-year sentence in the District Court of Newcastle, while his 26-year-old brother Craig Lambkin, who lit the fires, was sentenced to three-and-a-half-years.

In the District Court of Newcastle, Judge Tim Gartelmann noted that text messages between the brothers showed they had purposefully picked days when the mercury soared above 40 degrees.

"The degree of recklessness was very high," Judge Gartelmann said.

"Joshua Lambkin had a high degree of culpability in being a retained firefighter for encouraging Craig Lambkin to light fires, and would have appreciated the grave risk for many in community in doing that."

The fires, three of which were lit on January 18, with the fourth lit on January 24, forced the evacuation of homes, the local hospital, and a packed aquatic centre.

Houses like this property, photographed on December 18, 2017, caught alight and were damaged, but firefighters prevented any from being destroyed. ( Supplied: MJF Productions Australia )

Money not the motive: Judge

During the sentencing, the judge rejected evidence from the prosecution that Joshua Lambkin, employed on an on-call basis, wanted extra work so that he could get paid more.

Although Judge Gartelmann acknowledged that there was evidence that Joshua Lambkin was assigned to the fires and did receive payment, he was more swayed by the reasoning that Lambkin had hatched the plan simply because he "wanted to fight [the fires]".

Craig Lambkin admitted his guilt earlier than his brother, leading to a reduced sentence. ( Supplied: Facebook )

Craig Lambkin was seen on CCTV footage driving around the scene of the first three fires.

The court heard witnesses later told police that on January 24 they saw "a man riding a mountain bike" from the direction of the Kurri Kurri aquatic centre with black smoke billowing behind him.

The witnesses said the rider gave them a "cheeky grin" as he rode away.

Craig Lambkin was arrested shortly after and later admitted to lighting four fires at Loxford, Chinaman's Hollow, one near Kurri Kurri TAFE and another near Kurri Kurri hospital.

He pleaded guilty to four charges of causing the fires and being reckless to their spread, while Joshua, who initially denied involvement, pleaded guilty to several charges, including being an accessory before the fact, intentionally causing a fire and intentionally concealing a crime.

Smoke darkens the sky around Kurri Kurri on January 18, 2017. ( ABC News: Sarah Hawke )

Second firefighter implicated

Late in the trial the court heard fresh evidence about a second firefighter who may also have encouraged Craig Lambkin to light the fires.

That individual cannot not be named because of an active suppression order by the court, and it is understood the second named firefighter has not been charged.

In his pre-sentencing comments Judge Gartelmann said he had to consider the information on its merits and the impacts they may have on measuring Joshua Lambkin's level of culpability in directing his brother to start some of the fires.

Although he determined that the risk of reoffending for the pair was low, the judge criticised Joshua Lambkin's level of contrition, finding he was not completely remorseful.

In contrast he found Craig Lambkin had accepted his responsibility and impact on others, that he had admitted his guilt earlier that his brother, and was more truthful with police about his involvement, and in implicating Joshua Lambkin and another officer.

Craig Lambkin received a greater reduction than his brother in the final sentence.

With time already served he is likely to be released by August 2022, while Joshua's non-parole minimum period will see him in prison until at least August 2024.