Charles Castle's letter, to his then six-year-old son Ron, is one of 200 letters chosen to mark the 200th anniversary of the postal service.

The Top 200 Australian Letters of a Nation reflect a narrative of Australia's social history.

Charles Castle died almost 25 years ago, but thanks to Ron - today a 70-year-old grandfather of three living in Figtree - his memory will live on.

Charles Castle joined the Royal Australian Engineers in 1940 before sailing with the 8th Division to Malaya, leaving wife Ina and son behind.

He was captured during the fall of Singapore to the Japanese on February 15, 1942, and taken to Changi.

His first letter to Ron was dated September 13, 1945.

"Dear Ron, well little pal, your Daddy is the proudest man in the world having received your letter and the photo. What a big man you are now. It is now four years since I saw you and Mummy, but I hope to be with you soon. What a great time you, Mummy and myself are going to have ... Cheerio Ron, all my love to you and Mummy from your Daddy and Cobber always."

Charles arrived home in 1946 and met his family at Ingleburn Army Camp, a day Ron remembers vividly.

"They brought the boys in on buses," he said.

"They were all in uniform, but by this time they'd put on a bit of meat, they weren't the emaciated POWs we knew they had been.

"I'm standing there and he didn't even know who I was. He was expecting a baby."

Mr Castle said his father made up for lost time, both with him and the two children who followed - sister June McLean of Figtree and Trevor Castle of Farmborough Heights.

"He was a very good dad, but he had his down times because they were terribly maltreated in those POW camps.

"He quite often came home trembling, thinking the Japanese were chasing him."

Mr Castle submitted two letters for the collection, to honour his army sapper father and his servicemen colleagues.

"The sappers and the privates don't get a great deal of recognition and I would like to think that I've probably given Dad and his mates a little bit of credit," Mr Castle said.

Six letters connected to the Illawarra are in the Australia Post line-up.