About 45 guests gathered at the Kooringal Hotel on Saturday afternoon to share a birthday cake made of cupcakes arranged to form '100'.

Mrs van der Rijt said she was very happy to see all her family members again for her birthday.

Mrs van der Rijt's son, Adrian, told the party that turning 100 was a "huge feat" and it was incredible to think of the events and changes since she was born in the "roaring twenties".

Originally from Holbrook, Mrs van der Rijt moved to Wagga and ran the North Wagga Post Office for 17 years with her husband Jan and three of her five children still live in Wagga.

"The shop was open seven days a week and only closed on Christmas Day and Easter Sunday," Adrian said.

"This was a prodigious work effort for two people to put bread and butter on the table, as was the saying at the time."

Letters played a big part in Mrs van der Rijt's personal life as well as her professional life.

Adrian said Mrs van der Rijt was "known as a prolific writer" of letters, journals and family history.

"Combining her love of engaging with friends and writing in her early life, Mum had an extensive network of 'pen friends'," he told the party.

"She was diligent at writing letters to her many friends, including girlfriends, farmers, circus animal trainers, missionary priests and scientists working on pest animal control and snake antivenom."

Adrian said Mrs van der Rijt first saw her future husband when she traveled to Melbourne to meet a pen friend who was on the first ship of Dutch immigrants to arrive after World War II.

The family proudly displayed messages of congratulations including from the Queen and Prime Minister Scott Morrison.

Adrian said the family was "really lucky" to have the party as restrictions eased but some relatives in Victoria were unable to attend.