Christmas Island's small regional hospital cannot cope with an influx of medical transfers from Manus Island and Nauru, the local council warns.
Prime Minister Scott Morrison announced the detention centre will reopen to deal with any new asylum-seeker boat arrivals, and medical transfers, which arose from the parliament passing new transfer laws contrary to the government's wishes.
But the local council chief executive David Price says the tiny island is not ready to take asylum seekers with complex medical needs.
"If a person has a compound fracture they're air-vacced out. There's no operations done (here)," Mr Price told ABC Radio in Perth on Wednesday.
"Some of these people would have serious mental problems that need to be dealt with by specialists. We haven't got the specialists here to do that.
"We just wouldn't have the capacity to deal with people coming here for medical reasons, both physically and mentally."
He said it would make more sense to transfer the patients to mainland Australia in the first place.
Asked if the government gave the council any warning about the decision, Mr Price replied: "No, no, no, absolutely not."
"I think it's been a political kneejerk reaction by the government to the passing of that legislation by the Senate today."
The detention centre was officially closed in October, after Malcolm Turnbull promised at the 2016 election to shut it.