He was given two years to live at the age of 21. For the next 55 years he defied the odds -
and astounded the world with his theories on the cosmos.

The world's greatest scientist - and one of its finest minds - Professor Stephen Hawking, died peacefully at the age of 76 earlier today (NZT) at his home in Cambridge, north of London.

In a statement, the physicist's children, Lucy, Robert and Tim said: "We are deeply saddened that our beloved father passed away today.

"He was a great scientist and an extraordinary man whose work and legacy will live on for many years.

"His courage and persistence with his brilliance and humour inspired people across the world.

"He once said, 'It would not be much of a universe if it wasn't home to the people you love.' We will miss him forever."

In 1962, when Stephen Hawking turned 21, he was diagnosed with a degenerative motor neuron disease similar to amyotrophic lateral sclerosis (ALS), or Lou Gehrig's disease.

Doctors said it would probably kill him within two years.

Soon afterwards, he went to Cambridge to start a PhD in cosmology. The next two years were the most difficult of his life as he struggled to find a suitable doctorate subject while the crippling disease took hold.

But, as Hawking's body shut down, his mind began to expand new theories about black holes that would change our understanding of the cosmos.

When the disease was diagnosed, doctors gave him 14 months to live.

He was confined to a wheelchair by the time he was 30, and in 1986, aged 44, his voice was removed to save his life after an attack of pneumonia.

From then on, he spoke through a computer synthesizer on the arm of his wheelchair.

"I am quite often asked: how do you feel about having ALS?" he once wrote. "The answer is, not a lot.

"I try to lead as normal a life as possible, and not think about my condition, or regret the things it prevents me from doing, which are not that many."

The best-known theoretical physicist of his time, Professor Hawking wrote so lucidly of the mysteries of space, time and black holes that his book, A Brief History of Time, became an international bestseller, making him one of science's biggest celebrities since Albert Einstein.

His early life was chronicled in the 2014 film The Theory of Everything, with Eddie Redmayne winning the best actor Academy Award for his portrayal of the scientist. The film focused still more attention on Hawking's remarkable achievements.

Some colleagues credited that celebrity with generating new enthusiasm for science.

Hawking married Jane Wilde in 1965 and they had three children, Robert, Lucy and Timothy.

Jane cared for Hawking for 20 years, until a grant from the United States paid for the 24-hour care he required.

Hawking divorced Jane in 1991, an acrimonious split that strained his relationship with their children. Writing in her autobiographical Music to Move the Stars, she said the strain of caring for Hawking for nearly three decades had left her feeling like "a brittle, empty shell".

Hawking married his one-time nurse Elaine Mason four years later, but the relationship was dogged by rumours of abuse.

Police investigated in 2004 after newspapers reported that he'd been beaten, suffering injuries including a broken wrist, gashes to the face and a cut lip, and was left stranded in his garden on the hottest day of the year.

Hawking called the charges "completely false". Police found no evidence of any abuse. Hawking and Mason separated in 2006.

In May last year, Hawking warned that humans should leave earth within 100 years if we wanted to survive.

He believed that life on Earth was at an ever-increasing risk of being wiped out by a disaster, such as asteroid strikes, epidemics, over-population and climate change.

As our world became less suitable for life over the next century, Professor Hawking would warn in a documentary that future generations must forge a new existence in space.

As part of a documentary Professor Hawking planned to travel around the world to find out how we might exist in outer space.

In the documentary, Hawking would claim that time was running out for human existence on Earth and we must carve our future elsewhere.

In November last year, Professor Hawking was more conservative in his estimates.

He warned that humans could not survive another 1000 years on "fragile" Earth.

At a talk, Professor Hawking gave a one-hour whirlwind history of man's understanding of the origin of the universe from primordial creation myths to the most cutting edge predictions made by 'M-theory'.

He said: "Perhaps one day we will be able to use gravitational waves to look back into the heart of the Big Bang."

Auckland University head of the department of physics Richard Easther said he had met Hawking for lunch a couple of times and had met him at Cambridge University in the United Kingdom.

"In that sense he is a normal person, except he is not, he's Hawking."

Conversing with Hawking was an interesting experience, often Easther and other physicists would be talking shop and Hawking would chime in on something that had been said a minute or so ago, "ping-ponging" the conversation back.

"I knew him as a colleague. He was brilliant and was one of the top scientists in the 20th Century."

His primary contributions were understanding the connections between thermosdynamics, black holes and quantum mechanics, Easther said.

"His ability to see the linkages between these things was pointing us in the direction of fundamental theory of the universe."

His contributions would be studied for years to come, he said.

Hawking studied the complex nature of the black hole, using it as a "mathematical laboratory to understand the deep mysteries of the universe".

"He is a remarkable guy not for the science but how he lived his life," Easther said.

University of Otago department of physics professor David Hutchinson said he attended a workshop headed by Hawking in Santa Barbara in California back in 1999.

Although he was attending a workshop for ultra-cold atomic physics, he went to see Hawking speak about cosmology anyway

Hawking had an "Interesting sense of humour", Hutchinson said.

"It takes a long time for him to respond, it was a single word answer like 'no' then there was a pause then he would say 'just kidding' he was a humorous person."

He also had a connection to Hawking through his first honours student Dr James Fergusson who now works at University of Cambridge at the department of applied mathematics and theoretical physics where Hawking was a professor.

"He wasn't directly a student of Hawking but worked in the same group."

Prof Hawking was Britain's most famous modern day scientist, a genius with a razor-sharp wit who dedicated his life to unlocking the secrets of the Universe.

"My goal is simple," he once said. "It is complete understanding of the universe, why it is as it is and why it exists at all."

Much of that work centred on bringing together relativity - the nature of space and time - and quantum theory - how the smallest particles in the Universe behave - to explain the creation of the Universe and how it is governed.

In 1974, he became one of the youngest fellows of Britain's most prestigious scientific body, the Royal Society, at the age of 32.

Five years later, he was appointed Lucasian Professor of Mathematics at Cambridge University, where he had moved from Oxford University to study theoretical astronomy and cosmology.

A previous holder of the prestigious post was the 17th-century British scientist Isaac Newton.

Hawking shot to international fame after the 1988 publication of A Brief History of Time, one of the most complex books ever to achieve mass appeal, which stayed on the Sunday Times best-sellers list for no fewer than 237 weeks.

He said he wrote the book to convey his own excitement over recent discoveries about the universe.

"My original aim was to write a book that would sell on airport bookstalls," he told reporters at the time. "In order to make sure it was understandable I tried the book out on my nurses. I think they understood most of it."