A koala who was rescued by a topless grandmother during the New South Wales bushfires has been put to sleep because his injuries were 'too serious to treat'.

Toni Doherty was recorded running into an out-of-control bushfire in Long Flat, about 50 kilometres west Port Macquarie, to save the injured marsupial, named Ellenborough Lewis.

Lewis was taken to the Koala Hospital in Port Macquarie where he received treatment for severe burns to his hands, feet, arms and the inside of his legs.

But the koala's condition took a turn for the worse and the facility made the difficult decision to put him down on Tuesday.

Toni Doherty was recorded running into an out-of-control bushfire in the New South Wales town of Long Flat to save the injured marsupial, named Ellenborough Lewis

'Today we made the decision to put Ellenborough Lewis to sleep,' the hospital wrote on Facebook.

'We placed him under general anaesthesia this morning to assess his burns injuries and change the bandages.'

The koala hospital explained burns injuries 'can get worse before they get better'.

'In Ellenborough Lewis's case, the burns did get worse, and unfortunately would not have gotten better.'

'The Koala Hospital's number one goal is animal welfare, so it was on those grounds that this decision was made. We thank you for your ongoing support.'

The Facebook post was swiftly flooded with comments of support, as well as tributes to the koala's life.

'Collective hearts around the world have broken. Just as I'm sure those of the caring staff in the Koala Hospital already had. Thank you for doing everything you could to save this brave little man,' one person said.

'RIP little Lewis. Without the bravery of his rescuer and the treatment at KHPM he would have died in even more severe pain and suffering somewhere in the burning bush,' another said.

The koala's condition took a turn for the worst and the Koala Hospital at Port Macquarie made the difficult decision to put him down on Tuesday

The hospital released an update on Lewis's condition on Sunday, following numerous enquiries from concerned members of the public.

They said he was receiving substantial pain relief and 'round the clock care' but admitted the koala had an uncertain future.

'If we feel that his injuries and his pain are not treatable and tolerable, we will put him to sleep as this will be the kindest thing to do,' the hospital said on Sunday.

In the footage of Lewis's rescue, Ms Doherty could be seen pouring bottles of water onto the animal before scooping it up in a blanket.

Speaking to Channel Nine's Today Show on Thursday, Ms Doherty revealed why she decided to save the marsupial.

'It was vulnerable, all I could think to do was to try and rescue him,' she said.

'We just jumped out and I knew I needed to put something around him as I ran to the tree so I just took off my shirt and covered him with it and tried to get him out of the fire.'

Ms Doherty said she could feel the heat radiating off Lewis as he screamed in pain.

'He was being burnt. He was burning underneath, on his little back legs when he was approaching the tree, I've never heard a koala before, I didn't realise they could cry out,' she said.

Ms Doherty visited Lewis at the koala hospital while they were hopeful he would make a full recovery.

Ellenborough Lewis was named after Ms Doherty's grandchild.

Speaking to Channel Nine's Today Show on Thursday, Ms Doherty revealed why she decided to save the marsupial. 'It was vulnerable, all I could think to do was to try and rescue him,' she said