The precocious Joel Milburn claimed the huge scalp of Commonwealth champion John Steffensen to win a dramatic 400m gold medal at the Australian athletics championships.

Steffensen dominated the first half of the race, only for Milburn to step on the gas around the back bend and storm over the top of the pre-race favourite in the final straight to win in 45.90 seconds.

Steffensen was second in 46.17 and Sean Wroe finished third in 46.23 in the most competitive event at the championships.

The Olympic selectors now have several options, including naming all three of the place-getters in the Olympic team as soon as Sunday, or leaving any number of the 400m vacancies open until the June 23 cut-off date.

If Milburn, Steffensen and Wroe all get the nod from the selectors on Sunday, it will be cruel luck for three-time national champion Clinton Hill, who skipped the trials to be with wife Louise when she gave birth to their first child on Thursday.

Milburn's big breakthrough came when he slashed a second off his personal best in running 45.19 at the NSW titles last month.

"I've put in the work and I deserve it, I'm really happy with myself," he said.

Milburn insisted it had never been personal between himself and Steffensen, who he described as a mentor.

"It feels really good to be sort of equal to him, I'm really looking forward to racing overseas with him," he said.

"I knew John would go through the first 200 really quick and he's definitely got a really fast back straight, so I thought that I could probably win it on the third bend.

"I smashed that bend really hard and levelled up with him and from there on it's just who could hold on the longest."

The second-placed finish ended a summer of discontent for Steffensen, who did not race at all in the domestic series citing hamstring problems after making his displeasure very clear on being overlooked for Olympic preselection in November.

Discus thrower Dani Samuels and pole vaulter Alana Boyd guaranteed their berths in the Beijing Olympic team with victories, while Tamsyn Lewis's victory in the 800m ensured she would be named in that event on Sunday as well as the 400m.

Lewis's win in the 800m in 2:02.12 was her 13th overall national title and marked the second straight year that she had done the 400m-800m double.

The noted frontrunner successfully changed tactics, sitting in fourth place for the first lap before kicking away with 200 metres to go to finish well clear of Madeleine Pape, who is also on the fringe of Olympic selection.

"I want to be used to the competition, I wanted to be in a real race because that's what 800s are overseas, not me out front and everyone chasing," said Lewis.

"It's not like that because the girls are too good.

"I practised that tonight and I think I made a point that it is harder leading."

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