Leisel Jones has set three Australian records within half an hour at the national swimming championships in Sydney, including two in the one race.

The Olympic breaststroke silver medallist stunned swimming onlookers with a national record and an unexpected gold medal in the 200m individual medley after earlier setting two new breaststroke marks in her 100m semi-final.

The 17-year-old known as "Lethal Leisel" clocked 1m07.04s in the 100m breaststroke to establish herself as the third fastest swimmer over the distance in history.

Only world recordholder, South African Penelope Heyns, and China's Xuejuan Luo have gone faster than Jones's swim on Sunday night.

Jones hacked 0.27s off her old national mark set 12 months ago in the same pool and was just 0.52s off Heyns' 1999 world mark of 1:06.52.

Her time would have beaten American Megan Quann for the Olympic gold medal in Sydney two and a half years ago.

The Queenslander also established a new national 50m record during the race at the Sydney Aquatic Centre, clocking 31.54s at the halfway mark.

"Three Australian records in half an hour, that's pretty impressive," said Jones, who has improved her strength with increased weight sessions.

"I'm pretty happy with my breaststroke. It felt so relaxed, even felt a bit ordinary ... it just came naturally.

"I'm not predicting anything for the final. I think I did that last time and went not as fast as I wanted to go so I'll just stay relaxed, then everything should be fine, as long as I don't go slower."

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