Ronan Keating said Keith Duffy went into a "downward spiral" before being taken to hospital ahead of a Boyzone gig in Thailand.
Duffy was unable to perform at the show in Bangkok on Thursday night and Keating has said he is unsure whether he will be able to travel to Australia to join them for their next concert tonight.
Keating said Duffy (44) had undergone blood tests but they did not yet know the cause of his illness, speculating that it could have been food poisoning.
A statement from the band said Duffy had been taken to hospital "on the advice of the local medical team".
Keating added that he and Boyzone band-mates Shane Lynch and Mikey Graham had to leave Duffy in hospital to fly to Australia but that his wife Lisa was with him.
"He took ill before the show. He just went on a downward spiral," he said.
"He came to the gig. He had a massive, really high temperature.
"The doctors arrived at the gig and they said it wasn't possible for him to go on in the state he was in. We really don't know - we assumed it was food poisoning or something like that.
"But we don't know. There may be something else going on. We're waiting to hear back on the blood tests and whether or not he can travel.
"So we're hoping he can be on a plane tonight in Thailand and be here for the show tomorrow, fingers crossed," he added.
The Irish boyband, who are on their farewell tour to mark their 25th anniversary, are scheduled to perform in Perth tonight.
Keating said he hoped Duffy would be right for their Adelaide show next Tuesday if he cannot make the gig in Perth.
Asked about the Bangkok gig without Duffy, Keating said it was "hilarious".
"We had considered certain things but not others. I had to take some of Keith's vocals and I didn't know the words," he said.
"It was in When The Going Gets Tough and I had no idea, no clue whatsoever.
"I just looked at Shane and handed the mic out to the audience and hoped they'd sing along."