The dominant winner in Melbourne last season, Leclerc will line up for the race in seventh after lapping 0.637s off the pace of polesitter Max Verstappen, while Sainz will start from fifth.
While Ferrari has opted against introducing upgraded aero parts for this weekend, unlike Mercedes and Red Bull, Leclerc still shouldered responsibility for the lacklustre result, saying he "wasn't driving well" before being held up behind Sainz.
Leclerc said: "Q1 and Q2, clearly, I wasn't on it. I wasn't driving well. I wasn't putting everything together, so that was my fault.
"Q3, I managed to fare a bit better. In the car, I was quite confident I could put everything together.
"Unfortunately, I don't know what happened in the second run of Q3, whether it was a miscommunication with Carlos or whatever.
"But I found myself behind him for the whole first sector, which wasn't great. We'll speak at the debrief about that to try and improve those situations."
On a session interrupted by a red flag for Sergio Perez skating into the gravel while the threat of rain forced a rushed run plan in Q3, Leclerc added: "It was quite tricky, but to be honest, I feel like we are quite competitive. But I just didn't put everything together also in the first run of Q3.
"So, it's a bit my fault because when you arrive to Q3 you need to put everything together in whatever laps you do.
"We could have optimised it by having better communication. But I also didn't do a great job today."
Leclerc did report that Ferrari was in no worse shape than how it turned out in Saudi Arabia, which means keeping pace with Mercedes and Aston Martin in the race should be feasible.
He said the troubled qualifying was instead a one-off, after losing 0.2-0.3s that might have put him third or fourth but for driver error, meaning there was no need for Ferrari to "panic".
Leclerc continued: "I'm not particularly happy about the way I drove.
"I wouldn't panic in a way of today. I think two, three tenths. I just needed to drive better.
"The car wasn't that bad. The feeling was actually quite good. Part of the changes we've done this weekend was to make the race pace better."
Sainz, who rued a poor preparation lap when he was caught in traffic and allowed his tyres to cool too much, reinforced that the team had placed the emphasis on Sunday.
He said: "I'm reasonably comfortable. I think we've done some good progress with my feeling with the car.
"We've changed quite a lot, the set-up up and down from yesterday, trying to improve it mainly for the race."
