The Kiwi was working in the jungle of Corcovado National Park when he was bitten by a fer-de-lance snake.
Larisa Kellett, spokesperson for Bethune's non-profit Earthrace, said his condition deteriorated and he had to be carried out of the jungle.
"He was with a couple of other rangers and had to journey out of the jungle which was a challenge in itself because it's very dense.
"At first he could stand and move but once the venom got a hold of him he had to be carried out to a beach where a boat came and picked him up."
Kellett said he was taken to hospital where he was given anti-venom.
"I had an initial chat with him not long after it happened but it was obvious he was struggling and in excruciating pain, his speech was impaired so I was only able to chat to him for a moment," she said.
"He has made it through the night - they've given him anti-biotics but yeah his body is just working really hard to fight off the venom."
It's not the first time Bethune has been injured doing his work - in 2017 he was stabbed in the chest in Brazil.
