"I now have an eight-point advantage, which is pretty solid," said Smith afterwards.
"I beat Kurtis in every race. It was close racing and I had a few scary moments today, but, thankfully, no crashes.
"I thought I was going to have a really big one, but lucky was on my side today," said Smith.
Nelson's Hayden Wilkinson (Yamaha) remains third in 125cc, but is too far back to catch the leaders.
Smith will still have to perform well in the final round of the series at Taupo on March 25.
Overall points:1. Hayden Smith (Taihape, KTM) 213; 2. Kurtis Lilly (Karaka, Husqvarna) 205; 3. Hayden Wilkinson (Nelson, Yamaha) 154; 4. Dan Bell (Levin, KTM) 143; 5. Ryan Gwynn (Kakaramea, Husqvarna) 122.