Jones missed his weekday morning show on Sydney radio station 2GB and Brisbane's 4BC on Monday and Tuesday.

His fill-in, Chris Smith, said on-air on Monday that Jones was "currently in hospital being treated for severe back pain," AAP reported.

"His doctors are working to establish exactly what the problem is. The end result is he'll be back as soon as he can but we're not sure exactly when that will be."

A Macquarie Media spokesman said the company was in "constant communication" with Jones and his health professionals about his condition.

He didn't know if Jones would need to have surgery, or when he might be back behind the microphone.

"He doesn't know, and we don't know."

Jones' hospitalisation coincided with the release of Sydney's latest radio ratings, which showed 2GB maintained its dominance.

Jones still leads in the breakfast slot, despite suffering a 1.5 per cent loss to fall back to a 17.5 per cent audience share.

Last February, Jones reassured listeners he was "in one piece and not going anywhere" after major back surgery and chronic pain forced him off-air for several months.

He had gone under the knife several months earlier after spending most of 2016 using a walking stick and painkillers just to get around.