After leading by as many as four goals through the opening period, the South Africans, who Plummer coaches on only a part-time basis, held a 16-14 advantage at the first break, forcing New Zealand coach Janine Southby into unexpected changes early on.
It wasn't until mid-way through the second spell that the Ferns were able to settle into any sort of rhythm, clawing their way back into the lead by halftime.
From there, the New Zealand side slowly pulled away, but they never really looked settled, as the gutsy Proteas niggled away all night.
After ushering in the new era with an impressive 65-39 drubbing of England in their opening Quad Series match-up at the weekend, last night's stumbles against South Africa has provided the team with a timely reminder of their potential weaknesses.
Perhaps with an eye to Sunday's Quad Series showdown with Australia, Southby tweaked her midcourt line-up tonight's match, shifting Laura Langman back to wing defence, with Shannon Francois getting the start at centre.
Langman has long been seen as a potential antidote to dynamic Diamonds wing attack Madison Robinson - Australia's key link into star shooter Caitlin Bassett. The Ferns veteran, who plied her trade for the NSW Swifts this season, matched up well against Robinson in their ANZ Championship encounters.
But previous attempts by the New Zealand side to move Langman to wing defence have suffered for a lack of a strong replacement to the athletic star at centre. Francois, who had a strong season with the Southern Steel, did not step forward as a likely contender last night.
She was pulled after an ordinary opening quarter, in which the New Zealand side struggled for any rhythm on attack.
The return of Langman at centre in the second spell helped provide a bit more punch on attack, helping shooters Bailey Mes and Ameliaranne Ekenasio settle into their work more.
Southby was also forced into changes in her defence end, with Jane Watson, who made her debut in the Quad Series opener, introduced to try and contain the Proteas shooter Lenize Potgiter.
Potgieter, who impressed in her first visit to New Zealand in last year's two hit-outs against the Ferns prior to the World Cup, again caused problems for the home side, with her height and mobility proving a double threat.
Watson once again made a strong impact for the Ferns collecting four intercepts and a couple of other handy pick-ups, raising the possibility she may get the start against Australia in Melbourne this weekend, capping off an incredible rise for the 26-year-old, who wasn't initially even included in the wider New Zealand squad.
The Diamonds will take on England in Adelaide later tonight.
