A worrying return of kidney troubles for former All Black superstar Jonah Lomu has brought a tide of support for the rugby legend.

Lomu is in a stable condition in a renal ward in Auckland, and was expected to remain in hospital overnight.

He was diagnosed with a kidney condition - nephrotic syndrome - in 1995 at just 20 years old, and received a transplant in 2004.

In recent years he staged a return to rugby and this year was preparing for a charity boxing match when he was admitted to hospital.

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Fans took to social media yesterday to ask after Lomu's condition and wish him a speedy recovery, while friends said their thoughts were with a man who had already been through many struggles.

Former manager Phil Kingsley Jones said "like everybody else in the world" he was wishing for Lomu to recover quickly.

"Jonah Lomu is a special person to us - not because he's famous but because of the person he is," Mr Kingsley Jones said.

"If you know him at all he's a great guy and we just wish him all the best. We hope he will come right - he's been through so much - and our thoughts are always with him."

All Black assistant coach Steve Hansen said the whole team were behind Lomu.

"[It's] really sad news. And obviously we don't know too much about it other than that he's in hospital. So the thoughts are with the big fella and hopefully he makes a speedy recovery."

Kidney Health New Zealand education manager Carmel Gregan-Ford said the condition Lomu had suffered from was one of many that could lead to kidney failure.

There were 2500 people in the country on dialysis and up to 400 on waiting lists for kidney transplants, Ms Gregan-Ford said.

"It's just luck of the draw - any member of the general public could be at risk."

Nephrotic syndrome was characterised by the loss of protein in urine, and in many cases a new kidney should make it go away, she said.

"People with transplants can last a year to 20 years [or more]. It just depends on the kidney.

"With transplants there's always a risk of rejection, or somebody can get a virus or infection like in any part of the body. You can certainly have more transplants if one stops working, but you deal with the risks every time.

"There are more people donating kidneys than there used to be, but supply doesn't equal demand."

Simple blood and urine tests could detect many kidney problems, she said.