Hackett - regarded as one of the greatest long-distance swimmers - retired in 2008 but the 34-year-old Australian has been training under veteran coach Denis Cotterell on the Gold Coast since September.
The former world champion has stepped up his training in recent weeks with the national swimming championships looming in April.
Whether he goes on to compete for Australia remains to be seen as Hackett looks to get his life back on track after spending time in a United States rehabilitation facility for stilnox (a prescription medication used to treat insomnia) addiction.
"I probably haven't felt this good since 2003 and 2004," Hackett told News Corp.
"The most interesting thing is my body can still handle doing a fair bit of work which is great, that is why I think I've been able to get fit relatively quick.
"This year I've just really got my passion back for it. I do set myself little goals and little times and I'm trying to improve.
"Where it's going to lead I really don't know. My only ambition at this stage is to do a club relay with the boys."
Hackett previously dismissed talk of competing at the 2016 Olympic Games in Rio de Janeiro, though the veteran conceded things could change should he post a time worthy of selection on the Australian team.
"You would never say never in that case," he added.