Gregory Andrews started his protest in early November and vowed not to eat any food until Prime Minister Anthony Albanese "gets serious" and takes climate action.
But on the 16th day of his hunger strike, he reported feeling chest pains and shortness of breath before two nurses from Parliament House called an ambulance.
Mr Andrews was able to walk to the vehicle and entered with minimal assistance.
Nurses called an ambulance after Mr Andrews reported chest pains and shortness of breath.
In a blog post published a few hours before he was taken to hospital, Mr Andrews recorded his symptoms in detail.
"I'm losing my stability and finding it hard to breathe. My limbs and chest are aching all the time," he wrote.
"The Parliament House nurse who has been visiting me says that's because my body is starting to eat itself.
"I'm still doing my best but I'm not sure how long my 55-year-old body will hold out."
Mr Andrews has lost 10kg since the start of his hunger strike and prior to the ambulance call had indicated he would continue.
"I'm still determined to go for as long as my body allows or PM Anthony Albanese takes responsibility and commits to real action on climate change," he wrote.
The former diplomat worked with the government for more than 30 years, including as an ambassador to West Africa, threatened species commissioner and Australia's deputy chief climate negotiator at the United Nations.
He pointed to Environment Minister Tanya Plibersek's decision to approve four coal mine projects this year as fuelling his drive to take action.
The former diplomat is calling on the prime minister to take action on climate change.
Mr Andrews had planned to protest until at least December 13 in the hope Australia will ramp up its climate actions before the UN's COP 28 talks in Dubai.
Since beginning his strike, various politicians from the crossbench have paid a visit.
Greens senator David Shoebridge, independent senator David Pocock and independent MP Sophie Scamps have all met with Mr Andrews.
But his main targets have been silent.
"I still haven't heard anything from the prime minister or his office. He seems to be ghosting me. And Peter Dutton is missing in action," Mr Andrews wrote.
In his petition to Mr Albanese, the former diplomat called on the government to stop subsidising fossil fuels, commit to an urgent phase-out of coal and gas exports, end native forest logging, update environment protection laws and release key details of Australia's Climate Risk Assessment.
"Climate change is no longer a future emergency," the petition states.
"The science shows there's no more time left. The world is entering climate collapse and Australia needs to act."
Mr Andrews' petition has received more than 4500 signatures so far.
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