Skipper Adam Clarke started at inside centre but reverted back to halfback when Karaitiana Vette Welsh slotted into the team in the second half, and Mid Western coach Simon Cauty brought several sets of fresh legs on from the bench, opening the game up.
Centre Brooke Gilmore set the pace in the second 40, slicing through the Mid Northern defence to dot down two quick tries, putting the visitors on the back foot.
Despite fine performances by key figures Holwell, lock Buster Tahere and an overall solid effort up front, Mid Northern struggled to close the gap the hosts created.
Mid Western scored three tries to Mid Northern's three, but struggled in the kicking stakes, with their usual kickers - Luke Muggeridge and Ash Moeke - both away playing rugby overseas this season, Cauty said.
Dan Wells was in charge of the kicking duties on Friday night, with Clarke taking over later to successfully land two, but it is an area Cauty will look at more closely, along with his side's defensive efforts.
"Overall it was a pleasing start and quite a good game first up," Cauty said. "Mid Northern's big forwards just ran at us all day but it was our fitness that got us there ... we had a really good preseason."
In other round-one matches, Marist narrowly beat Hikurangi 15-13 while Kamo drew with Old Boys 15-15 at Kamo, Wellsford beat Western Sharks 13-10 and HoraHora beat Otamatea 58-12.