Dual Olympic champion Hackett, 35, has come out of six years of troubled retirement to try to make Australia's team for Rio next year, and has just spent three weeks training with Phelps under coach Bob Bowman in Arizona.
He also trained with American stars Ryan Lochte and Missy Franklin, who gave him a crash course in cutting-edge techniques in the sport, The Australian newspaper reported.
Hackett, who retired in 2008 after winning the 1500m freestyle at both the 2000 Sydney and 2004 Athens Olympics, and claiming four world titles in the event, said he found his time with Phelps particularly beneficial.
