McKenzie put the hosts ahead with another penalty a minute later. The Magpies regained the lead in the 25th minute when captain and hooker Ash Dixon scored from a lineout drive after he turned down an easy three points in front of posts.
Southland dominated possession and territory in the second quarter courtesy of sound option taking from McKenzie and halfback Logan Crowley. However, rock-like defence from the Magpies kept the Stags tryless.
Dixon scored his second try from a lineout drive just before Mahoney blew halftime. Second five-eighth Danny Toala converted to give the hosts a 17-6 lead at the interval.
Southland scored a spectacular try in the 42nd minute. McKenzie put a clever kick ahead which winger Lewis Ormond gathered in before sending centre Isaac Te Tamaki over to touch down. McKenzie slotted the conversion.
The third quarter was reasonably tight with both teams displaying plenty of endeavour. The Stags displayed better control than in previous weeks.
Hawke's Bay secured the bonus point for four tries in the 57th minute when halfback Folau Fakatava made a solo burst and fed Toala who dived over. This brought back memories of what the pair did for the 2017 national title-winning Hastings Boys' High School first XV.
Toala added the extras. Lock Michael Allardice in his first starting XV appearance of the season for the Magpies had a massive workrate. Fullback Caleb Makene deserved the player-of-the-match award as he read play well when combating the tactical kicking expertise of McKenzie.
Along with McKenzie and Crowley, Ormond, made the most of his opportunities in the Stags backline. Captain and blindside flanker Brayden Mitchell, away from his more familiar hooking position, was inspirational in the visiting pack.
Any hopes the Stags had of leaving the Bay with a bonus point were dashed when prop Morgan Mitchell was sinbinned in the 66th minute for a maul infringement. Substitute loosie Brendon O'Connor extended the Magpies lead with a try from another lineout drive a minute later. The Magpies led 34-13 after Fakatava sent his former HBHS first XV first five-eighth Lincoln McClutchie over for a 70th minute try after a clever overhead pass.
The Stags were rewarded for their never-say-die approach when fullback Josh Moorby scored in the 74th minute after more good work Ormond down the right hand touchline.
The Magpies were determined to have the final say and substitute prop Jason Long scored in the 77th minute. McClutchie slotted the extras. However it was appropriate the courageous Stags should fight back and scored the final try through Te Tamaki.
Match highlights
Heartstopper: Stags centre Isaac Te Tamaki's 42nd minute try. It hushed the 6000 Magpies fans temporarily.
Turning point: Magpies captain and hooker Ash Dixon's 39th minute try which gave his side a 17-6 lead after a period of sustained Stags pressure on the Magpies goal line.
Player of the match: Magpies fullback Caleb Makene. Positional play accurate on defence and made the most of his attacking opportunities.
Scorers: Hawke's Bay 41 (Gareth Evans, Ash Dixon 2, Danny Toala, Brendon O'Connor, Lincoln McClutchie, Jason Long tries; Toala 2, McClutchie cons ), Southland 23 (Isaac Te Tamaki 2, Josh Moorby tries; Marty McKenzie 2 pens, con ). HT: 17-6.