Mr Assange, who has been living in the Ecuadorian embassy in London since 2012, recently had his internet access cut off and has been banned from receiving visitors.
Anderson, who said she was a close friend of Mr Assange, told ITV’s Good Morning Britain she was “very concerned about his health”.
She said: “I think this is a form of torture. I think they’re slowly killing him. I’m very, very, very concerned, deeply concerned for him. He’s one of the most important people on the planet right now.
“He’s exposing all these war crimes and truths. We really need him at this moment.It’s not a coincidence he’s being investigated at this time.”
The Ecuadorian government decided to stop Mr Assange using the internet or social media from its embassy in London last month.
Mr Assange has been living there for almost six years, believing he will be extradited to the United States for questioning over the activities of WikiLeaks if he leaves.
Canadian model and actress Anderson was among those urging Ecuadorian president Lenin Moreno to reverse the ban on internet access and visitors, signing a letter along with the likes of musician Brian Eno and fashion designer Dame Vivienne Westwood.
The former Playboy model, best known for her roles on TV series Baywatch, Home Improvement and V.I.P., said she “wouldn’t be surprised” if her own relationship with Mr Assange was being investigated.
“Theresa May needs to do something,” she added. “The UK is being so stupid. They have to make a decision. They keep on passing the buck, no-one wants to take responsibility.
“They want to extradite him to the US, 100 per cent, and that would be life in prison or worse. Before something happens to him, someone needs to confirm that they’re not going to extradite him to the US. That’s all he cares about.
“It’s really important. There’s so much going on in the world right now. I want to know what he’s thinking, I want to know what he’s thinking about all this and he’s being silenced.”
Anderson said that his living conditions “with no sunlight, being in a small space for so long and now being cut off from everybody, is just inhumane. It’s a human rights situation, he’s being abused”.
“He’s not the culprit, he’s just the messenger.”