Triple Olympic gold medallist Grant Hackett will attempt to qualify for the national team almost seven years after walking away from the sport.

The 34-year-old will line up on the blocks against swimmers almost half his age when he competes at the Australian Swimming Championships which begin in Sydney on Friday.

Hackett said upheaval in his personal life led him back to the pool, but the distance champion said he is a different swimmer the second time around.

"I don't have expectations like I did before and I sort of reserve the right to pull out at anytime," he said

A lot of people are wondering why Hackett decided to return to competitive swimming almost seven years into his retirement.

"I sort of ask myself that same question when I'm training every morning and night," he said.

The distance champion has rejoined the Miami Swimming Club on the Gold Coast.

He is being coached by Denis Cotterell, who guided Hackett to gold medals in the 1500 metres freestyle at the Sydney Olympics in 2000 and again in Athens in 2004.

Cotterell said retirement had not diminished Hackett's work ethic.

"He trains like he used to and that's as hard as anyone can possibly go," Cotterell said.

The 34-year-old competed in two lead-up meets and clocked respectable times in the 200 metres and 400 metres freestyle events.

His coach says the former distance king is not contemplating a return to the 1500 metres, a race he dominated internationally for more than decade.

"It's a little too short a time frame," Cotterell said.

Hackett agreed that the 1,500 metres was out of his reach.

"If I really want to continue with this and if I look to going all the way to the Olympic trials next year .. it's just really too short a time frame," he said.

"For the time that I've actually had out of the sport and the fitness and the base work that actually needs to be done for a race like that, it would just be too difficult."

Grant Hackett's factfile 2000 Olympics: 1500m freestyle gold, 4x200m freestyle gold

1500m freestyle gold, 4x200m freestyle gold 2004 Olympics: 1500m freestyle gold, 400m freestyle silver, 4x200m freestyle silver

1500m freestyle gold, 400m freestyle silver, 4x200m freestyle silver 2008 Olympics: 1500m freestyle silver, 4x200m freestyle bronze

1500m freestyle silver, 4x200m freestyle bronze Former long course world record holder in 800m freestyle, 1500m freestyle, 4x200m freestyle.

The sport has changed since Hackett retired in 2008 with athletes now doing more specific training and an increased focus on nutrition.

The 34-year-old is now learning from younger athletes including the world-number-one 200m freestyle swimmer Thomas Fraser-Holmes, as well as rising 1500m swimmer Jordan Harrison.

"Overall it's actually the best squad that I have ever trained in so to just be alongside those guys and trying to be competitive every day is a real challenge for me," Hackett said.

Hackett said his comeback is different from former team mate and friend, Ian Thorpe, who returned to the sport after four year hiatus and failed to qualify for the London Olympics in 2012.

"I think for me I have come from a totally different avenue as opposed to what Ian did," he said.

"Ian was making a comeback really focused on the Olympics and it was probably a bit more grand than anything I was intending on doing."

Hackett said he is excited to be competing at the Australian Championships and it will allow him to gauge his place among the country's elite swimmers.

"Whilst I am enjoying it I will continue to do it but if I don't enjoy it, I can pull out at any time because it's not like I have set up any grand vision of this big comeback.

"I want to just enjoy swimming and hopefully swim fast."

Hackett said he will not entertain the prospect of competing at next year's Olympics in Rio.

"People talk about the Olympic Games but that is something I am not even thinking about," he said.