Having put City 2-0 up with two early close-range finishes, Haaland converted from John Stones’ header to make it nine goals in his first five Premier League matches – a new competition record.
Joao Cancelo then extended City’s lead further with a fine long-range effort, before Julian Alvarez opened his Premier League account with two more goals on his first start.
The dominant win ensured Pep Guardiola’s side maintained their unbeaten start to the new season and laid down an ominous marker to any would-be challengers, with the champions now fully up to speed.
Dean Henderson made a reflex save to deny Alvarez six minutes in, but he was rooted to the spot when Haaland poked Phil Foden’s deflected cross inside the near post to hand City a 12th-minute lead.
Renan Lodi headed into the side netting when left unmarked five minutes later, and the visitors were made to pay for that miss when Haaland played a fine give-and-go with Foden before rolling into an empty net.
The Norwegian then made it nine goals for the campaign to kill the contest as the break approached, taking advantage of some dire Forest defending to head home on the goal-line following a deep free-kick.
City picked up where they left off after half-time as Cancelo rifled into the top-left corner from the edge of the area, before Alvarez marked his first Premier League start with a goal by finishing low across Henderson when one-on-one.
Haaland was substituted to a standing ovation before Alvarez added his second late on, bringing down a loose ball before volleying into the top-left corner to add further gloss to the scoreline.
What does it mean? Haaland leads City to ominous start
City racked up 93 Premier League points despite spending much of last campaign without a recognised striker, and it looks increasingly difficult to see anybody taking their title following the arrival of Haaland.
His staggering return of nine goals in his first five Premier League appearances is a new competition record, surpassing the impressive eight-goal hauls of Sergio Aguero and Micky Quinn.
Goal machine keeps ticking over
Haaland’s second hat-trick in as many games saw him upstage former City transfer target Harry Kane in emphatic fashion.
The City number nine required just five appearances to bring up nine Premier League goals during the month of August – one more strike than Kane has managed during 26 such appearances.
City record resounding win
City scored five or more goals in a Premier League game for the 32nd time under Pep Guardiola on Wednesday, which accounts for 14 per cent of their games under him (32/233).
Guardiola’s men have hit teams for five more than twice as often as any other side since he joined the club, with Liverpool doing so in 15 occasions.
City travel to Aston Villa for their next Premier League game on Saturday, while Forest host fellow promoted side Bournemouth the same day.