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."
Source: Autosport