World record holder Grant Hackett's fitness will come under the microscope in Wednesday night's 800m freestyle final at the national titles in Brisbane.

The distance master pulled out of the 200m freestyle final on Tuesday night in a bid to rediscover form at the world championships trials at Chandler Aquatic Centre.

The 26-year-old won the 400m freestyle on Sunday but has been unable to hit his straps, with coach Denis Cotterell saying his taper was off.

It is Hackett's first major meet since shoulder surgery last year and the Australian swim captain is desperate to produce some strong performances after missing this year's Commonwealth Games.

He is the reigning world champion in the 400m, 800m and 1500m and has already qualified for the national team for next year's Melbourne world championships by winning the eight-lap event.

He could potentially bomb out in both the 800m and 1500m and still swim the events at the worlds so long as two other Australian swimmers don't beat him and meet the selection criteria in either event.

The qualifying times are 7:50.40s in the 800m and 15.11.94s in the 1500m and Craig Stevens looks like the only other Australian swimmer capable of meeting those times.

Australian head coach Alan Thompson has the discretion to allow members of the national team to fill vacancies in any events.

The other highlight is the women's 200m freestyle final with Bronte Barratt (1:58.73) the fastest qualifier, well ahead of Commonwealth record holder Libby Lenton (2:00.22).

Meanwhile 14-year-old Emily Seebohm, who rose to prominence by deadheating in the 100m backstroke on Tuesday night, continues her campaign in the 50m backstroke heats on Wednesday morning.

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