The former New Zealand international Jonah Lomu remains in an Auckland hospital on Thursday amid growing concern about his health.

The 36-year-old was recently admitted to Auckland City Hospital's renal and transplant unit. He underwent a kidney transplant in 2004 after the New Zealand radio host Grant Kereama became his donor.

Lomu had been diagnosed nine years earlier with a kidney disorder known as nephrotic syndrome. Friends and former playing colleagues are said to be concerned, although it has emerged that he has signed an exclusive contract with New Zealand Woman's Weekly to report on his stay in hospital.

The Auckland District Health Board said in a statement: "The Lomu family has asked that ADHB not release any information whatsoever about Jonah's care. We have respected their wishes at all times and continue to do so. The same courtesy would be extended to any of our patients.

"However, ADHB has become aware of the possibility of an exclusive agreement with a magazine. ADHB has had no involvement in any agreement whatsoever.''

Lomu's last public appearance was during the World Cup opening ceremony at Eden Park, Auckland, on 9 September, when he played a starring role.

The New Zealand World Cup squad, currently in Wellington for their pool game against Canada on Sunday, stopped their Wednesday training session to send Lomu a photograph message.

Players and staff held up letters spelling: "Get well, Jonah. Kia kaha (be strong)."

The All Blacks hooker Keven Mealamu told the New Zealand Herald: "We just want him to get better. He has been a big part of the All Blacks in the past and probably a big part of what they are today, so it's just a nice message from the boys hoping he gets better quick."

Lomu, a juggernaut 6ft 5in wing, scored 37 tries in 63 Test match appearances for the All Blacks after making his debut in 1994.