The 22-year-old bagged a 26-minute first-half treble to put Forest to the sword, just four days after doing the same to Crystal Palace.
He is now the first player in the Premier League era to score nine goals in his first five games, with many backing the Norwegian forward to better Andy Cole and Alan Shearer's record high of 34 goals in a season.
Speaking after the match, Haaland told BT Sport: "It was about keeping on doing what we did second half against Palace. We knew there were going to be chances and that's exactly what we did. It was amazing. It's been good so far and I'm not complaining.
"I try to listen as much as I can to Pep (Guardiola) and to play with the boys. They are amazing footballers and without them I would not be able to score goals. It's about finding chemistry, playing together, and to listen to Pep.
"That's why I came here. We play so good, we find each other, and it will become even better. It's important to keep going because there are going to be a lot of games."
This was also a story about another new City striker as Julian Alvarez marked his first Premier League start with a brace after the break, with Joao Cancelo also scoring.
The hammering took City's goals-for column up to 19 in five games and highlighted just how hard they are going to be to stop this season.
Forest, who have shown enough already this term to suggest their return to the top tier will not be a fleeting one, will be happy to get this fixture out of the way early on and will view Saturday's clash against Bournemouth with far more relish.
Haaland was less than two years old when Forest, who his dad used to play for, last visited Manchester City in the league and City are an unrecognisable club from the one that won that second-tier clash 3-0 back in 2002.
They have dominated the Premier League in recent years and are an even more frightening prospect with Haaland now spearheading the attack.