In the closing laps of the Melbourne race Alonso was left battling Russell for sixth place.
Fearing how vulnerable he would be through Albert Park's DRS zones, Alonso claimed he tried to brake earlier for Turn 6 on the penultimate lap so he could carry more speed onto the following straight.
Mercedes man Russell was caught out by his manoeuvre and crashed off in the Aston Martin's dirty air, with the stewards investigating any erratic driving by the two-time F1 world champion.
After considering the evidence and an explanation from Alonso, the Spaniard was handed a drive-through for "potentially dangerous" driving, which was converted into a 20-second penalty post-race.
The penalty dropped him from sixth to eighth in the final results and promoted team-mate Lance Stroll to sixth and RB's Yuki Tsunoda to seventh.
Telemetry showed that Alonso had lifted 100 metres earlier than usual and tapped the brakes. The stewards deemed that he slowed down to an "extraordinary" extent, more than reasonable to optimise his corner exit.
Alonso said he was disappointed by the verdict because he didn't feel he did anything dangerous at all.
"George caught me quickly, I knew that he was coming," Alonso gave his view after being demoted to eighth.
"Then he was in DRS range for five or six laps, so I was just doing qualifying laps to stay ahead.
"I wanted to maximise my exit speed from Turn 6 to defend against him. That’s what any racing driver would do, and I didn’t feel it was dangerous.
"It’s disappointing to get a penalty from the stewards for what was hard but fair racing. Still, I’m glad that George is okay. It was not nice to see his car in the middle of the track."
Later Alonso double down on his views on social media: "A bit surprised by a penalty at the end of the race regarding how we should approach the corners or how we should drive the race cars.
"At no point do we want to do anything wrong at these speeds. I believe that without gravel on that corner, on any other corner in the world we will never be even investigated.
"In F1, with over 20 years of experience, with epic duels like Imola 2005/2006/Brazil 2023, changing racing lines, sacrificing entry speed to have good exits from corners is part of the art of motorsport.
"We never drive at 100% every race lap and every corner, we save fuel, tyres, brakes, so being responsible for not making every lap the same is a bit surprising.”
Team boss Mike Krack said he was surprised by the stewards' verdict, but Aston Martin wouldn't appeal the decision.
"It was surprising to see him drop to P8 with the post-race penalty, but we have to accept the decision," he commented.
"This will not distract from a positive day. We have scored 12 championship points.
"Full credit to both drivers, who converted our grid positions into points, making use of two very different tyre strategies."
