The Campos driver found himself in potential hot water mere moments after the lights went out on Saturday in Albert Park.
As he pulled to the right to pass polesitter Roman Stanek, he failed to notice his fast-starting team-mate Pepe Marti and caused the Spaniard to collide with Gabriel Bortoleto.
The latter pair made heavy contact with the wall and retired on the spot, while Hadjar continued to take the top spot on the podium.
But a post-race investigation resulted in the stewards imposing a 10-second penalty on the Frenchman, dropping him to sixth and awarding Stanek his first F2 victory.
Feature race polesitter Dennis Hauger is therefore promoted to second, while Kush Maini completes the top three.
Speaking before facing the stewards, Hadjar said: “It [the start] was quite stressful. I had a good launch and then I tried to go for the move on Roman.
“I didn’t see what was going on on my right-hand side and I just had contact. That was it really.”
Asked by Autosport if his push to secure a 6.8s margin at the front had been to negate any damage inflicted by a potential penalty, he added: “I didn’t think about the penalty at all during the race, I just drove my pace and we will see what the stewards say.
“As I said, in the car, I didn’t see what happened. I was just focused on taking the lead, so we’ll see.”
As well as being handed the penalty, two points were also placed on Hadjar’s licence.
Source: Autosport