Australia reacts to unlikely Rio gold

As Australia celebrated Chloe Esposito's shock Olympic gold on Saturday morning many were also scratching their heads, questioning what event she had won.

Social media lit up as Esposito secured Australia's first ever modern pentathlon medal, with more than a few people learning for the first time what the event entails.

"This weird event came on where they were running between witches hats on the equestrian course, and then they started shooting guns mid run," one eloquently posted on Facebook.

"What an Aussie hero! Chloe Esposito swims, fences, rides a horse, shoots and runs. She has to carry our flag at closing ceremony," another tweeted.

The modern pentathlon involves a 200m swim, fencing bouts and a 12-jump show jumping course before the athletes complete a shooting event dispersed over three 1km.

Former tennis great Todd Woodbridge, surfer Sally Fitzgibbons and NRL star Jarryd Hayne were among several Australian athletes to join in the celebration on Twitter.

"How good! What a sport. Congrats Chloe Esposito," Hayne said.

"The unexpected golds are always the best! You're a champion Chloe Esposito," another supporter tweeted.