The Wallabies have lost their opening two matches of this year's Rugby Championship to the All Blacks, following up a 38-12 defeat in Sydney with a 40-12 reverse in Auckland last weekend.
Michael Cheika's side had been in both matches going into the second half, but they went on to concede a combined total of 59 points after the restart, prompting certain sections of the media to claim their drop off was down to a lack of fitness.
Rebels star Haylett-Petty has dismissed those claims, however, and insists the losses were more down to the mental side of the game, as Australia have only won two of their last 21 meetings with the All Blacks.
"I don't think we're not fit enough, no," Haylett-Petty told Rugby Australia's website. "It's something we need to go back over the game plan whether we could work smarter, not harder. I don't necessarily think it's a fitness thing.
"If anything, [it's] concentration, we let ourselves down and you see other teams capitalise."
The defeats have left Australia bottom of the Rugby Championship standings, but they will get a chance to return to winning ways when they take on South Africa in Brisbane on September 8.