The Matildas' Olympic campaign has got off to an awful start following a 2-0 loss to Canada at Arena Corinthians.

Australia endured a disastrous start as a defensive mistake saw Janine Beckie steal in to slam home the opener after just 20 seconds of play - the fastest goal in Olympics history.

Canada was reduced to 10 players after a straight red card for Shelina Zadorsky, and the Matildas proceeded to dominate the rest of the half thanks to the numerical advantage.

But instead of tiring in the second stanza, Canada improved and won a penalty following a handball in the box from Clare Polkinghorne.

Matildas goalkeeper Lydia Williams came up big for Australia, saving Beckie's tame penalty.

But Canada deservedly doubled its lead through star forward Christine Sinclair, who beat the stranded Williams to the ball to slot home into an empty net from distance.

Despite having 22 total shots on goal to the Canadians' eight - and 12 shots on target compared to four - Australia could not manage to get the finish it required to get back in the game.

Striker Michelle Heyman said the team did not do enough with the player advantage.

"It is a bit unlucky playing against 10," she said.

"[I'm] a bit disappointed in myself I think I created a couple of good opportunities and I should have put them away."

Coach Alen Stajcic and his side will have it all to do as the Matildas' next opponent is world number two side Germany on Sunday morning (AEST).

"It is heartbreaking and devastating the result of today's match," Heyman said.

"But I think we will have a little think about it and try and forget about it tomorrow so we can just try and focus on the next game."

Germany is in the driving seat of Group F after beating Zimbabwe 6-1.