He was more than two seconds off his world record but Ian Thorpe, clad in a sleek "new beaut suit", won an unprecedented sixth straight Australian 400m freestyle title in Sydney.

Thorpe clocked three minutes 42.41 seconds to maintain his five-year dominance over Grant Hackett, who was again relegated to a minor rung of the medal dais by the swimming superstar.

The 20-year-old Olympic and world champion's winning time at the Sydney Aquatic Centre was way off his world record of 3:40.08 set at last year's Manchester Commonwealth Games.

But it still ranks as the ninth fastest 400m freestyle ever swum and a time never bettered by any other swimmer.

"I was a bit disappointed with the time. I was happy with the swim and how I felt during it but I looked up and I thought I would have gone a bit faster," Thorpe said.

"I think I may have had an off-swim but it's a good gauge of where I am at and what I can do at the world championships."

The eight-day national titles are doubling as the selection trials for the world championships in Barcelona in July.

Thorpe, who wore a modified version of his neck-to-ankle suit, remains unbeaten over eight laps since his win as a 15-year-old at the 1998 Perth world championships.

The all-black suit was recently tested in a wind tunnel belonging to car manufacturer Audi to improve its fluid dynamics.

"It's entirely different to what I have worn. This has panels on the back that actually work in with the flow of water behind your body and reduces a bit of drag there," Thorpe said.

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