The 36-year-old was recently admitted to Auckland City Hospital's renal and transplant unit. He underwent a kidney transplant in 2004 after New Zealand radio host Grant Kereama became his donor.
Lomu had been diagnosed nine years earlier with a kidney disorder known as nephrotic syndrome.
No official statements have emerged from the hospital about Lomu's condition, but friends and former playing colleagues are said to be concerned.
Lomu's last public appearance was during the World Cup opening ceremony at Eden Park, Auckland on September 9, 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 training session on Wednesday to send Lomu a photograph message.
Players and staff held up letters spelling: "Get well, Jonah. Kia kaha (be strong)."
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.
Regarded as the sport's first true global superstar, he showcased his immense skills during the 1995 World Cup semi-final against England in Cape Town, when he scored four tries, famously "running over" Mike Catt for one of them.
He played in New Zealand for the Hurricanes, Chiefs and Blues, while he also had a short playing stint in Wales with Cardiff Blues.
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