The rampaging All Black winger was rushed by ambulance to hospital on September 23 after struggling from nausea and weight loss, and is now on daily dialysis in hospital.
A kidney donated to him in 2004 was believed to be shutting down, which meant his body was struggling to clear waste from his system.
A week after his admission to the hospital's renal ward, the health of his transplanted kidney was still uncertain.
Waitemata District Health Board renal specialist Johan Rosman said dialysis would be a hugely debilitating undertaking for Lomu.
An ordinary-sized person suffering from kidney failure needed to have dialysis five hours a day, three times a week.
Because of Lomu's bulk he would face five sessions of treatment a week, four hours at a time.
Of Lomu's condition, Professor Rosman said: "It can be two things: acute rejection, which is less severe and happens all of a sudden, and it can be chronic rejection, which you see coming for a while.
"If it's chronic rejection, then the outlook is more grim. The sheer fact that he is on dialysis is not a good sign."
If Lomu's body rejects his transplanted kidney, he might face a wait of six months to eight years for a new donor, depending on how rare his blood type is. There are also added complications in a second transplant, because the body must re-adjust to accept another foreign organ. Dialysis treatment would prevent Lomu from participating in sports.
He is still scheduled to box for three rounds as part of the Fight for Life charity event in December.
Promoter Dean Lonergan said he would not make any changes until Lomu's condition was certain.
Medical experts have dismissed the suggestion that training for the bout had contributed to his poor health.
