Following the second red flag period initiated by the right-rear tyre breaking free from the Haas of Kevin Magnussen, Sainz clipped Alonso during the race restart on the exit of the Turn 1 right-hander.
Two-time F1 world champion Alonso had lined up third for the third standing start of the heavily disrupted Melbourne race with his compatriot alongside on the second row of the grid.
Alonso moved to the outside of the track after an initial run at Lewis Hamilton but as he cut back across, the front-left wheel of Sainz's car tagged the AMR23 to send it spinning into the wall.
The controversial handling of the resulting third red flag - with a safety car called to lead the field over the line in the order of the previous restart minus crashed cars - allowed Alonso to reclaim third.
However, Sainz was handed a five-second penalty and dropped to a point-less 12th as a result of the bunched-up train of cars crossing the line in quick succession.
Alonso reckoned the punishment did not fit the crime, saying: "Probably the penalty is too harsh.
"On lap one [of a restart], it is very difficult always to judge what is the grip level, and I think we don't go intentionally into another car, you know?
"We know that we risk also our car and our final position, so sometimes you ended up in places that you wish you were not there in that moment.
"It's just part of racing. I didn't see the replay properly, but for me, it feels too hard."
Sainz labelled his reprimand as "the most unfair penalty I've ever seen in my life", with Ferrari team boss Fred Vasseur frustrated that his driver was not given a hearing to give his version of events before being punished.
Alonso gave his account of the collision, saying: "Obviously, I didn't know who touched me in that moment. I just felt in lap one of the first start, someone touched me in Turn 3.
"Then in the last restart, Carlos apparently touched me in Turn 1. I mean, probably only Jeddah [previous round in Saudi Arabia] I had the normal start.
"In Bahrain also in Turn 4, Lance [Stroll, Aston team-mate] touched me, so I'm very attractive out there!
"But our car is strong enough. So, it doesn't matter if they keep touching us."
