Caroline Buchanan qualified fastest in the women's competition, but the strong Australian team had a mixed start to BMX cycling at the London Olympics.
Buchanan showed the form that took her to this year's world time trial championship when she clocked 38.434 seconds for the 440m outdoor course.
She was .210 of a second faster than New Zealander Sarah Walker, the 2009 world champion.
Buchanan's compatriot Lauren Reynolds was ninth at 1.611 seconds and British star Shanaze Reade qualified fifth at .934.
American Brooke Crain crashed and did not gain a qualifying time, but she will compete in Friday's semi-finals.
Dutch rider Raymon van der Biezen was the only rider in the men's field to go under 38 seconds for their 450m time trial.
He clocked 37.779 seconds, .442 up on French star Joris Daudet.
Reigning world champion Sam Willoughby was the top Australian in sixth at .717.
Khalen Young was 17th at 1.525 seconds and Brian Kirkham qualified 22nd at 1.831.
Latvian Edzus Treimans crashed heavily in his run, but like Crain went through to the next round.
All three Australian men are through to their Thursday quarter-finals, with the men's and women's medals to be be decided on Friday.
The qualifying times decide the seeding and gate selection order for the first run of the men's quarter-finals and also for the first semi-final run.
The same applies for the first run of the women's semi-finals - they do not have quarter-finals.
In the men, there are four quarter-final heats of eight riders each.
Each heat has five races, or runs, and the top two after the first three rounds go through to the semi-finals.
The remaining riders have two more runs and the top two also go through to the semis.
The men's and women's semi-finals are held over three runs, with the top four from the two heats going into the final.
The final to decide the medals is a single race of eight riders.
The Australian and British riders were unavailable for comment after their seeding runs.
The Australian Olympic cycling team policy is that riders do not comment to the media while their competitions are ongoing.