The Australian had looked competitive earlier in the weekend but admitted the picture shifted heading into qualifying as rivals made gains.
Piastri briefly threatened a place on the front two rows late in Q3 before slipping to fifth as improvements from Isack Hadjar and Charles Leclerc pushed him down the order.
His teammate Lando Norris ended the session one place further back in sixth.
“I think after yesterday probably painted an overly optimistic picture for us, but we felt like we were in the mix. And after FP3, we definitely didn’t feel like we’re in the mix,” he said.
“So yeah, after FP3 by far the biggest thing was straight line speeds. I don’t know if it’s the same after qualifying, but I think that’s going to be something we need to understand.”
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Piastri, whose McLaren is powered by a Mercedes engine, had topped Friday’s second practice session at Albert Park with a lap of 1:19.729s after earlier power issues disrupted his running in FP1.
However, the home favourite slipped back in final practice, finishing eighth as Mercedes surged to the front.
That swing became clearer in qualifying, where George Russell showcased the true pace of the Mercedes package in 2026, claiming pole position ahead of teammate Kimi Antonelli.
Piastri said the expectation within the paddock had been for a closely fought session, but Mercedes ultimately made the decisive step forward.
“I think our thoughts and the picture of it being that everyone was pretty close. And then Mercedes jumped ahead and the rest wasn’t pretty close,” he said.
“I think to me the biggest thing was we didn’t seem to gain very much for us. So it didn’t seem to gain very much through qualifying.
“I don’t know if that was car or driver.”
The Australian also suggested the opening qualifying session of Formula 1’s new regulatory era had exposed how difficult it can be to extract performance from the current machinery.
“Clearly the way you naturally want to go fast doesn’t work. But maybe there needs to be more restraint looked into,” he said.
Asked about the driving experience itself, Piastri offered a brief verdict: “It’s different.”
He said much of the challenge stemmed from the complex energy management required under the new systems, which can leave drivers short of power at various points of the lap.
“Everyone can see the state of things,” he said.
“I think it will probably improve a bit. But there’s clearly some fundamental things that won’t be very easy to fix.
“And I don’t really know what we do about that.”
Piastri also explained that some circuits are already highlighting those challenges more than others depending on how energy is deployed through a lap.
“It’ll be better at different tracks,” he said
“We’ll have different challenges at other tracks because the tracks are kind of in two categories at the moment of being energy starved and energy rich.
“But I think when you’re energy starved like this, it’s a lot more obvious to everyone watching.”
He said the increased complexity had also added to the workload in the cockpit, with drivers needing to constantly monitor their steering wheel displays.
“We’re looking at the dash a lot, yes,” Piastri said.
“And I’m sure everyone’s seen how we have to start a qualifying lap now. Which isn’t great.
“And there’s just a lot of things you have to do to optimise.”
The Australian Grand Prix takes place at 3pm AEDT Sunday.
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