Bethune is the founder of non-profit Earthrace, which does conservation missions, such as tracking illegal fishing and mining.
Earthrace spokeswoman Larisa Kellett said Bethune had been bitten on the calf by what is believed to be the highly venomous Fer-de-lance snake and is now in Costa Rica's Golfito Hospital.
The bite left the conservationist with extreme swelling and the "worst pain he's been in".
Kellett said one single bite from fer-de-lance snakes has enough venom to kill 32 people.
The snake bit Bethune while he was doing conservation work in a jungle on Costa Rica's Osa Peninsula, Kellett said.
Bethune had to crawl out of the jungle, navigating several waterfalls while his condition worsened, she said.
"He said he was dragging on his bum at one point," she said.
"For the last few hundred metres he was carried as he was in extreme pain."
The Kiwi conservationist was transferred by Zodiac to the Earthrace and taken to hospital in Golfito Bay, she said.
"We do not know if he will be okay. He is in the best care that he could have. He is in excruciating pain. His voice was slurring a lot. He wasn't speaking as he was normally."
Bethune is under close supervision of medical staff and might be moved to another hospital to access specialist care, she said.
New Zealand-based Kellett wasn't sure what type of work he was doing, but guessed he was patrolling for illegal gold miners, which involves following trails to find the miners.
"You get in there. You get your backpack and your gear to last a few days. It's harsh conditions. You look for trails to see where the gold miners have made their little camp. They put chemicals in the waterways to do the actual mining."
Kellett said she spoke to Bethune on Christmas day. She then received a WhatsApp message from one of Bethune's crew members advising that their captain had been bitten by a snake.
"It happened today. We're not out of the woods."
