The Australian logged the sixth fastest time in the three-part qualifying session which would have seen him line up alongside Charles Leclerc on the third row.
However, a three-place grid penalty for Sergio Perez means he’ll instead start fifth, with the Red Bull Racing driver alongside him.
“It’s been a pretty solid weekend so far,” Piastri said when asked about his qualifying session by Speedcafe.
“Q1 and Q2 looked strong. Q3, I mean, first and foremost, I made too many mistakes, and that was the biggest reason of why I am where I am.
“I think we could have done a couple of things a bit better in terms of out lap preparation too, but there was also my input in that out lap preparation as well.
“Just too many mistakes,” he reiterated.
“I feel like in terms of position, I was somewhat fortunate that I’m not further back.
“Not a terrible result, just a bit disappointing because I know there was more on the table.”
McLaren will start the Australian Grand Prix among the Ferraris and Red Bulls at the pointy end, with Lando Norris to line up third behind Max Verstappen on pole and Carlos Sainz.
The two leading teams are expected to edge clear in the race, with McLaren’s battle realistically with Mercedes for best of the rest honours.
The Brackley-based squad had a torrid session, struggling to get the tyres working.
George Russell could do no better than seventh and Lewis Hamilton only 11th, with the Aston Martin duo of Lance Stroll and Fernando Alonso ninth and 10th.
“It’ll be tough,” Piastri said of his prospects of moving forward.
“I think we looked pretty strong in our race runs yesterday, but the cars around us also looked strong, or at least the cars in front of us.
“To beat Red Bull and Ferrari, we’re going to need to find something extra or have a bit of good fortune.
“If we can finish around where we start and not go further backwards, that would honestly be a good day for us.
“We’ll try our best. It’s optimistic stuff, I know, but I think that’s just where we sit at the moment.
“And I think it’s been, honestly, a good step forward in terms of we seem to be a bit quicker than Mercedes and Aston [Martin] around here, which is better than we have been in the last two rounds.”
The Australian Grand Prix begins at 15:00 AEDT tomorrow.
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