The Ferrari driver tangled with Fernando Alonso at the second race restart of a chaotic Australian GP, sending his fellow Spaniard into the wall at Turn 2 before the third red flag.
Amid the FIA's final restart decisions, Sainz lined up for the final lap in fourth but was given a five-second penalty for the incident, so when the race ended under safety car conditions he was relegated to 12th when the penalty was applied.
An angry Sainz wished to review the penalty with the stewards, stating over team radio: "No, this is unacceptable! They need to wait until the end of the race to discuss with me."
Sainz later felt he needed to see the stewards to talk through the incident and penalty before giving his full take on it.
"I think it is the most unfair penalty I've ever seen in my life," he said after the Albert Park race.
"Before talking to you and saying any really bad stuff or bad words, I'd prefer to go back to the stewards, have a conversation with them and maybe I can come back and talk again.
"Because right now honestly I cannot do it, I think it is too unfair and I don't feel well to speak."
Ferrari team boss Fred Vasseur agreed with his driver and felt the stewards were too hasty, given the call wouldn't have impacted the podium result.
At the last race in Saudi Arabia, the stewards came under pressure for handing Alonso a post-race penalty which changed the order, promoting George Russell onto the rostrum after the podium ceremony – only for the penalty to be reversed on review.
"My frustration is more that they took a decision before the end, before they listen, with one lap behind the safety car," he said.
"It would've made sense to wait for the hearings and discuss exactly what they thought and what they see. It wasn't going to change the podium at all, it was not a drama."
Sainz was also handed two penalty points on his license after the stewards found he was "wholly to blame for the collision".
"Car 14 was significantly ahead of Car 55 at the first corner and nevertheless Car 55 drove into Car 14, causing it to spin and leave the track," stated the stewards' document.
"For avoidance of doubt, we took into account the fact that this collision took place at the first lap of the restart, when, by convention, the Stewards would typically take a more lenient view of incidents.
"However, in this particular case, notwithstanding the fact that it was the equivalent of a first lap incident, we considered that there was sufficient gap for Car 55 to take steps to avoid the collision and failed to do so."
Sainz's penalty left Ferrari without any points from the weekend. Team-mate Charles Leclerc went off on the opening lap after clashing with the other Aston Martin of Lance Stroll at Turn 3, which was judged as a racing incident.
The result leaves the Italian manufacturer trailing in fourth place in the F1 world constructors' championship, and already 97 points behind leaders Red Bull after three races.
