Tests will determine whether rugby star's failing transplanted organ can be revived.

Dialysis will buy Jonah Lomu as much time as needed for his kidney to be revived, or to get a re-transplant.

He's in Auckland Hospital's renal ward, with doctors reportedly saying there's a slim chance his donated kidney will survive.

Professor of Transplant Surgery at Auckland City Hospital Stephen Munn says donated kidneys don't last forever.

He says there's no time limit for a person to be on dialysis as it's the next best thing to having a fully functioning kidney.

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"People who are on dialysis are not as well as people with a functioning transplant but they can certainly buy time that way.''

Professor Munn says donated kidneys don't last forever as they undergo a slow chronic form of rejection.

"Having a second transplant is not unique by any stretch of the imagination, probably 10 to 15 percent of our transplants are re-transplants.''

He says with about 600 people on the list for a kidney, the average wait time is three years, unless someone offers a live kidney transplant.

Lomu, a former left wing, was rushed to hospital just days after starring in the Rugby World Cup 2011 opening ceremony.

He was donated a kidney in 2004 by ZM radio host Grant Kereama.