100-year-old war veteran Wattie Thomas' may have slowed down in recent years, but his smile still lights up the room and his mind remains sharp.

The Invercargill man celebrated his centenary with family and friends on Sunday 16 October with his three sons Trevor, Neil and Roy among those by his side.

Thomas struggles to get around without a wheelchair and his hearing is poor, but he still has a good memory, his sons say.

He reminds them of things they did 60 years ago when they cannot remember themselves.

"He remembers things yesterday and what he did 60 years ago," Neil said.

Thomas, the secretary of the Southland Farmers Co-op for many years and a leading figure in the RSA, was offering up no fancy reasons for longevity.

Instead, he put it down to a healthy lifestyle and being surrounded by loved ones.

"Good living and a good family to look after," he said.

His appetite remained large and his preferred tipple was scotch and milk - a war-time habit.

"He can put a plate of food away like no-one I have ever seen," a grandson said at the family gathering to celebrate Thomas' birthday at the Bill Richardson Transport Museum on Sunday.

Born in Dunedin, Thomas moved to Invercargill in 1937 and has lived at the Rowena Jackson Retirement Village for the past nine years.

In between times he fought in World War 2 - he is understood to be the only living person in Southland to have the RSA Gold Star for exceptional service.

As a 22-year-old he was shipped off to Egypt and Syria for four years.

Speaking about that experience last year, he said he never thought he would see New Zealand again.

"We lived by the day."

When not in battle they would play games such as cribbage and visit the Turkish border when they had leave.

During the war years Thomas would write letters to his love, Ito, and they later married and had three sons together.

Ito died 14 years ago.

Over the decades Thomas was heavily involved in the RSA and lawn bowls and he grew huge vegetable gardens for the family dinner table.

Despite never catching up with the computer age, he took his first selfie at age 99, with the help of a grandchild.

Family friend Bob Simpson said Thomas was a lovely guy and a gentle man.