Error prone Bay of Plenty found their best form late in the game, but any hope of victory was ended when Daniel Waenga was unable to convert Toby Arnold's 76th minute try.
The home side led from start to finish in a superb effort dominated by their determined pack and a policy of spreading the ball wide.
They even survived the second half sin binning of Ash Moeke, whose early jersey pull on Lelia Masaga went dangerously close to conceding a penalty try.
Bay of Plenty, still out of sorts at that stage, were unable to make Northland pay for being a man down.
Bay of Plenty brought on leading forwards Tanerau Latimer and Culum Retallick at halftime and filtered on other top rankers during the second spell but could not turn the tide.
Northland got off to a flying start and led 24-10 at the break after spreading the Bay of Plenty defence from one side to the other, and attacking down the flanks in numbers.
Halfback Rhyan Caine's break set up Bryce Heem's first try and lock Mark Burman scored the second when centres Aaron Bancroft and Derek Carpenter created a hole in the Steamers' defence.
Sam Cane smashed his way over the line, after Moeke dropped a kickoff, which reduced Northland's lead to seven points.
But Northland charged down the right hand touchline to set up Bryce Heem's second try on the left as halftime approached, with Munro landing the wide conversion.
