ADELAIDE (AFP) - Dual Olympic champion Grant Hackett struggled into fourth place, as Mack Horton swam a scorching time to win the 400 metres freestyle at the Australian Olympic swimming trials in Adelaide on Thursday.

Hackett, 35, who came out of six years of troubled retirement to try to make the Games, could not keep pace with Horton and David McKeon and missed out qualifying in the event for the Rio Olympics.

Horton's winning time of 3min 41.65sec was the seventh fastest of all-time and the second-fastest by an Australian behind Ian Thorpe.

McKeon finished second in 3:45.09 to also qualify for Rio, while Hackett came in fourth in 3:48.84.

Hackett, who retired in 2008 after winning the 1,500m freestyle at both the 2000 Sydney and 2004 Athens Olympics, and claiming four world titles in the event, will now attempt to qualify for the Olympics in the 4x200m freestyle relay.

"Those two guys were well ahead of me and I was always realistic about this race and the fact that I had missed a lot of work and I will now be focusing on the 200m swim," he said.

"Just to be in that race and see Mack swim a 3:41 is an incredibly fast swim and it was awesome being in that race.

"That was thing about today whether to race tonight and make myself a little bit tired for tomorrow and at the end of the day I'm here to challenge myself and certainly did that tonight and will do that over the next couple of days.

"The focus has always been on the 200m freestyle, hey, I'm 36 next month, let's not kid ourselves, I've been around for a while and these guys are the stars of today and it's great racing here and to be competitive."

In other finals, Thomas Fraser-Holmes dominated the 400m individual medley in 4:11.09, more than three seconds faster than nearest rival Travis Mahoney.

Blair Evans (4:35.26) and Keryn McMaster (4:37.94) qualified for Rio in the women's 400m individual medley.