Omanu's Max Beattie then also took silver in the men's board race with Midway's Cory Taylor not too far behind in sixth.
McMillan then took silver in the men's surf race.
Australian beach flags champion Chanel Hickman (Mairangi Bay) then took gold in the women's beach flags, her first medal for the competition.
Teammate Paul Cracroft-Wilson followed up with a medal-winning performance, taking silver in the men's beach flags.
Not content with two golds and a silver already under her belt from Rescue 2014, McKenzie then stepped in to win another silver for the team in the ironwoman with Midway's Cory Taylor also taking silver in the iron man.
The women's ocean relay team of Natalie Peat (Papamoa), Natasha Hind (Lyall Bay), McKenzie and Hickman then claimed the day's only bronze medal.
In world champion style, the team finished the day as they started - on a high note, winning gold in the men's taplin team of Taylor, McMillan, Cracroft-Wilson and Beattie.
The New Zealand Youth Team also had a successful day in the pool on their final day of competition, picking up another two medals to add to a successful campaign.
Papamoa's Ben Johnston and Sumner's Brittany Tucker each picked up bronze medals in the men's and women's 50m Manikin Carry.
Johnston finished in a time of 31.96 with Germany's Joshua Perling in first place in a time of 31.28 and Spain's Domingo Togores in 31.88.
Tucker clocked 37.77 behind Australia's Prue Davies in a time of 37.32.