An elderly couple has died after bushfires swept across northern NSW earlier this week.

Authorities have confirmed human remains were found on Thursday inside a home at the Coongbar property of a 77-year-old man and 69-year-old woman. A number of residents are still missing.

The fire destroyed at least eight houses in Drake on Tuesday while 21 homes were lost in the neighbouring Busbys Flat fire, the Rural Fire Service said on Thursday afternoon.

Authorities have also said they believe the fire that ripped through the Rappville area this week may have been deliberately lit, in what the NSW emergency services minister, David Elliott, has described as a truly “bastard act”.

The Long Gully fire is believed to have been started by a lightning strike and is not being treated as suspicious.

The RFS commissioner, Shane Fitzsimmons, said the deaths were a “tragic, horrible” outcome.

“[It’s] a truly sobering reminder of the ferocity of these dangerous and destructive fires that we have seen burning across northern NSW for months now,” he said.

Superintendent Toby Lindsay won’t say exactly how many people are still unaccounted for.

“We are worried about a number of people,” Lindsey told reporters on Thursday in Rappville where at least 11 homes were lost.

“We always hope for the best but we’ve obviously got to treat it at its worst. We’ve seen the devastation in town. It’s a very large fire and it’s still active.”

Richmond Valley mayor Robert Mustow asked locals to report into authorities.

“We have still got some people missing and the longer that goes on the more concerning it gets,” Mustow said.

“I would ask people, if you’ve vacated your property, please go down to the evacuation centre (and) register,” the mayor told ABC TV.

Mustow said the loss of a local timber mill would hurt 30 employees and have a “big impact on our community”.

NSW police and fire investigators have formed a strike force to determine the cause of the fire.

“In the absence of any other obvious cause, we always default to being suspicious, and we work through a very thorough process,” the NSW Rural Fire Service commissioner, Shane Fitzsimmons, told the Nine Network. “We’ve got to call it out – it’s a heinous crime, a criminal act.”

Across the Busbys Flat and Drake fires - which have joined together in recent days and burned 115,000 hectares - at least 21 homes have been destroyed. Fifteen were in the Rappville area while the remainder were impacted by the Drake blaze.

Further buildings have been damaged but the Rural Fire Service says numbers will rise as more properties are inspected.

Fitzsimmons said 34 bushfires continue to burn across NSW, eight of which remain out of control. However, all fires are currently at an “advice” alert level after a number of emergency warnings were downgraded on Wednesday.

Allan Robertson lost his Rappville home and was left with the clothes he was wearing, his phone and “my worst thongs”.

He said he was uninsured and his partner was in palliative care.

“It was just like a massive fireball,” he told 10 News. “There was nothing you could do. The heat was horrendous.”

Another resident, Brayden Leschingkhol, told Seven News it was more like “hell from fire ... not from this world”.

He became emotional as he recalled calling his mother, who lives in Newcastle, saying: “I told her I love her and basically told her goodbye.”

The Brisbane to Sydney rail line has been cut and won’t be operational again for an estimated five days.

NSW Transport Minister Andrew Constance tweeted that stretches of the rail line had been damaged and the Rappville rail bridge destroyed.