From there Athletic began to find their way into attacking positions more often but without reward.
City regained the lead minutes before halftime when standout Latham Berry slotted one passed the King.
Despite being down, Athletic may have been the happier side at the break and they had their best period when they came out for the second half.
King kept them in touch with a handful of quality saves but he and his side's luck ran out when City's Jordan Joblin-Hall pounced on a parried save to extend the lead.
Saunders continued to be Athletic's brightest on attack but couldn't find the back of the net before City finally put the game to bed in the 85th minute when Kyle Graham-Luke tapped in to make it 4-1.
City coach Kelvin Francis was happy with the result overall but said they missed a lot of the chances they created.
"Their keeper played out of his skin. He kept them in the game to be honest. If it wasn't for Heath I think it might've been double figures," Francis said.
"But I was really happy. We played exactly the way I wanted us to play. We came out, pushed the ball around and dominated the game.
"We talked at halftime about the fact that they were walking in the first 20 minutes so we knew that we needed to keep the pressure on and that would open up the spaces later."
Athletic's Andy Lendrum said it was a disappointing result given they'd kept themselves in touch for much of the game.
"We thought their first goal of the second half really hurt us because we had 20 minutes of good footy and we were really pushing them back and then suddenly they scored and we got out heads down and let the flood gates open unfortunately. Up until that point we were in it," he said.
"We've got a young team, it was a bit of a shock to play at that speed. It was pretty hard and physical but on the whole a good derby. Plenty of intensity from both sides and plenty of heart - but gutting."
Athletic score first win of Federation League
Over on Wembley 1 Devon Homes Wanganui Athletic notched their first win of the Lotto Federation League season beating Massey 1sts 4-0.
A second-half brace to Ryan Holden and goals from Scott Burney and Symon King contributed what coach Jason King said was a pleasing performance following a frustrating opening two rounds.
"The boys dug deep, did the job, kept calm and made the most of the opportunities we had," he said.
"The finishing was good. We still obviously need to make more of the opportunities we are creating but it's still early in the season, the guys are still getting used to each other, but overall I'm a happy man."
The win lifts Athletic to third on the ladder ahead of a trip to Gisborne next week.