Breen broke the Australian record for the women's 100 m sprint, when she clocked 11.11 seconds at the ACT Championships, held on 9 Feb 2014 at the Australian Institute of Sport track in Canberra under ideal conditions with a 1.9 mps following wind, warm conditions and 600+ metres elevation.

This broke a record previously held by Melinda Gainsford-Taylor, which had stood for more than 20 years.

Her personal best time in the 100 m event is 11.11 seconds set on 9 February 2014 in Canberra.

Her personal best time in the 200 m event is 23.12 seconds, set on 9 March 2013 in Sydney.

[5] At the German competition, she had an opportunity to qualify for the Olympics in the 4 × 100 m relay, but she and Australia's other runners did not make the top 16.

[5][7] Breen was one of fifty-four track and field competitors chosen to represent Australia at the 2012 Summer Olympics.

[6] The chairman of Athletics Australia justified her selection, saying "In the case of Melissa Breen, at our final selection meeting, like all eligible athletes, we took into account her season’s results and progression over the qualifying period.

With multiple personal bests, an improved level of consistency in her performances and the fact that she is an emerging athlete in our sport, the selectors chose to identify her for nomination to the AOC.

"[7] Breen was the only athletics competitor in Australia who was nominated who had not set an Olympic A qualifying time.