Joao Cancelo set the champions on their way to a routine win before Phil Foden grabbed his fifth goal in three matches. Riyad Mahrez made it three early in the second half before Haaland scored his customary goal to make it four.
Southampton faced the uneviable task of trying to stop a City side that had struck 20 goals in the past five matches in all competitions and of course Haaland, who went in search for a fourth successive home hat trick in the league, but had to settle for just the one this time.
"The result is good, we play quite good. We struggled to break their high pressing. Last season we suffered a lot against them but today we were much better," City boss Pep Guardiola said.
"Some players were not at their best but we create a lot of chances. We were solid. It was a good game.
"For a long time Phil Foden is playing really good. The impact from him is great.
"Erling Haaland had the chances again. He scored a brilliant goal. It's an incredible weapon for us, a big threat. I am so satisfied.
"We will see the result from Arsenal and Liverpool, always in Premier League there are good games.
"Being close to the top of the league is important going into the World Cup."
Haaland hit the post inside the opening 15 minutes before City broke the deadlock through Cancelo, who found the net after a fine solo run. Foden doubled the lead on 32 minutes when he was afforded far too much space and he made no mistake, chipping the ball over Gavin Bazunu.
Southampton made it to half time with just the two-goal deficit but four minutes after the restart, the gap widened further as City started to take control. Rodri's pass found Mahrez and he volleyed in for City's 44th goal of the season -- quite the feat in the first week of October.
Guardiola took goalscorer Foden off with the game won, though the Etihad crowd grew increasingly surprised as Haaland failed to find the target. They need not have worried, though, as his goal eventually came on 65 minutes to make it 20 for the season.
"It was a difficult game, we made it easy by scoring early and then we controlled. We had a lot of chances to score more but we didn't. We didn't concede much, it was a controlled game," Mahrez said.
"Haaland is part of our team, he is our striker. Like any other striker, if we can find him it's perfect because he is on fire. If not, there are a lot of players and everyone can make a difference. We have to keep going with the momentum."
Southampton boss Ralph Hasenhuttl said: "The quality you are facing, this is what I was expecting. They changed their shape and we had more problems today to press them high.
"They showed the difference between these two teams. I am not unhappy with the effort and bravery we showed. Not with the ball though, we played too risky. The problems in the first half came after losing the ball in possession.
"We made a step back, possession is not usually an issue. But overall we tried to stay brave and give the players something to work on.