Subzero has "made some improvement as he continues treatment for serious illness but his long-time carer Graham Salisbury says the popular horse is still not out of the woods.

The 1992 Melbourne Cup winner who turns 31 on Thursday, remains at the Bendigo Equine Hospital where he has been since last week.

After overcoming a bout of colic, he has been treated for kidney issues and also peritonitis.

"He's found a half-length over the weekend," Salisbury told RSN927.

"He's sort of improved that little bit.

"He is nowhere near out of the woods yet.

"They'd love to be able to say you could come down and pick him up and bring him home, but (vet) Sarah is very very confident that he has come another half-length, that he's found a little bit, which is a good sign.

"His temperature is still up a little bit, that's what she's a little bit concerned about. And she's a little bit concerned that he is not passing any manure just yet.

"But there's a bit of oil that they've pumped into him, that's starting to come through so it could be a good sign of what's to follow.

"At this particular point of time he's comfortable. He's a fighter.

"Fingers crossed that he's on the repair."

After he retired from racing in 1994, Subzero forged a new career as Salisbury's mount as clerk of the course at Flemington.

He has since endeared himself as a racing ambassador and regular visitor to children's hospitals and nursing homes and in 2014 Subzero was honoured by the Australian Racing Hall of Fame for his services to the community.