Manyweathers finished three strokes back at four under after an even 72 wasn't enough to knock Clarke off his perch.
Curnow ended in third, well off the leading pace with an even par 216 after going one under in the last round.
The best round of the 54-hole men's championship was shot by Hayden O'Donnell (Tauranga) in his opening gambit.
His five-under round gave him the lead heading into the final two but inconsistency and a poor final round of a nine-over 81 left him six over for the tournament.
Bryan Toohil (North Shore) won the best net, and Neil Weber (Omanu) the best stableford.
Meanwhile, the chase was on in the final round of the Women's Mid-Amateur with a team of women's masters' representatives trying to catch Kellie Orchard from Royal Wellington.
Orchard started the day with a two-shot lead over defending champion Shelley McElroy (Springfield) with Waikato's No 1, Jill Morgan (Cambridge), lurking a further shot behind.
Five shots off the pace was Vaha Fapiano, who shot an impressive final round of 71 today, the best round overall for the women, to make it within one.
Fapiano finished on 227 (77, 79, 71) with McElroy on 231 (77,76, 78).
PaR nz Golfing Holidays, which manages the tournament, confirmed the 2019 New Zealand Mid-Amateur will return to Golf Omanu for the third consecutive year.