UPDATE 4.23pm: SAM the koala was a symbol of Victoria's resilience after the bushfires and her death is 'tragic', Kevin Rudd says.

Sam, whose image was seen around the world in the aftermath of the Black Saturday bushfires, was expected to undergo surgery today to treat cysts as a result of urogenital chlamydiosis, a life-threatening disease that affects 50% of the koala population.

This afternoon, vet John Butler told waiting media outside Morwell Veterinary Clinic that due to complications with the disease, the best course of action was to put Sam down.

Prime Minister Kevin Rudd led the national mourning for Sam, describing her as an enduring symbol of Victoria’s resilience in the face of the Black Saturday fires.

“It’s tragic that Sam the koala is no longer with us,” Mr Rudd said.

The PM was informed of Sam’s death this afternoon during talks in Cairns with Pacific Island leaders.

“I just heard this before I came in,” Mr Rudd told journalists.

“Very interesting, when I travelled to London for the G20 summit in early April, the symbol of hope for so many people around the world was the great picture of that wonderful koala being fed water by one of our firefighters.

“I think that gave people of the world a great sense that this country, Australia, could come through those fires as we have.

“And Sam the koala was part of the symbolism of that and it’s tragic that Sam the koala is no longer with us.”

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• Your thoughts: Leave your tribute to Sam below
• Listen now: Vet on the decision to put Sam to sleep
• Pictures: Sam's story | See the video that started it all
• More: Royal Commission | Interactive | Bushfire reports

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David Tree, the CFA volunteer who featured in the iconic image of Sam giving her a drink of water, was distraught when the Herald Sun broke the sad news.

"It's like not worth it now, that she managed to survive the fires but die from a disease that they should be able to vaccinate her against," he said.

"I'm sobbing like a baby and I am a grown man.

"She meant so much - she highlighted the plight and vulnerability of Australian wildlife around the world.''

Mr Butler said that due to "extensive changes" in the condition of bladder and uterus this morning, the decision was taken to put Sam down before surgery to prevent her suffering too much pain.

“Dr John Butler of the Morwell Vet Clinic operated on Sam shortly after 1pm today and discovered that Sam had severe changes in her urinary and reproductive tract that was non-operable and unfortunately had to put Sam to sleep," said Peita Elkhorne of TressCox Lawyers.

"The changes were consistent with the Chlamydia infection. It was so severe that there was no possible way to be able to manage her pain.

"Colleen Wood and her team at Southern Ash Wildlife shelter and all of those who have been involved with Sam are devastated with this loss."

In a statement, Southern Ash Wildlife Shelter said they were grateful for all the support they have received and will continue to provide aid and comfort to distressed and injured wildlife.

Sam captured hearts across the world in photos and video that helped to raise hundreds of thousands of dollars for the CFA after the Black Saturday bushfires.

Southern Ash Wildlife Shelter spokesman Nic Pullen said yesterday the turn of events was extremely distressing.

"Sam was making good progress with her recovery, her burns had healed and she had even developed a strong maternal instinct taking care of some of the orphaned joey's admitted to the shelter over the past several months,” Mr Pullen said today.

“The discovery of cysts due to chlamydia is very upsetting for everyone involved in Sam's recovery."
Up to 50 per cent of the koala population is affected by the disease, with past vaccines proving unsuccessful.

Sam’s image was flashed around the world, while a video of David Tree crouching down to offer the furry marsupial a gulp from his water bottle is among the most watched videos on heraldsun.com.au.

The pair became accidental symbols of the devastating bushfires when Mr Tree found an exhausted Sam in the middle of burnt-out bush in Mirboo North.

Donations can be made to Southern Ash Wildlife Shelter here