Dressage rider Mary Hanna was on Friday unveiled as Australia's oldest ever Olympian with the 61-year-old grandmother selected to compete in Brazil.

She will also become only the third Australian woman to represent her country at five Olympics, alongside table tennis player Jian Fang Lay, who is also heading to Rio, and retired beach volleyballer Natalie Cook.

"Every time I have done the Olympics I've thought, this is probably the last time I will do it," said Hanna, whose first Olympics was in Atlanta in 1996.

"But after the last time I thought, 'I am going to keep going with this because I feel fit and healthy and why shouldn't I'? So, here I am."

Hanna will be 61 years and 253 days when she competes on August 10 to surpass Bill Roycroft, who also competed in the equestrian event in Montreal in 1976 when he was 61 years and 128 days.