Helensville's Tom Buxton won round one of the series near Whangamata in March, Cambridge rider Dylan Yearbury clinched the win at round two near Porirua in April and Thames rider Chris Birch topped the podium at round three near Tokoroa two weeks later.
But it was Reardon who coped best with the slippery conditions near Martinborough on Sunday.
"It was certainly a good day for me," enthused Reardon afterwards.
"I was suffering a bit because I'd hurt my shoulder at another event the previous weekend – cracked my collarbone actually – and so I was riding very cautiously at Martinborough.
"But that worked in my favour anyway, because the conditions were so slippery that this was exactly what was needed to get the job done.
"I was in a bit of pain, but didn't notice it too much when I was riding.
"I'm pretty excited to be leading the national championships. I have raced before at the Masterton venue, it's forestry and pretty technical, but I like that."
Manawatu's Paul Whibley, a two-time former cross-country champion in the United States, finished runner-up on Sunday, with Eketahuna's Charlie Richardson claiming the third podium spot.
Reardon won all four of the day's high-speed 'special tests', but ended the full day's racing just 26 seconds ahead of Whibley, with Richardson only another 47 seconds further back.
"It was a beautiful day, perfect weather throughout, although the soil was somewhat soft," said Justin Stevenson, Motorcycling New Zealand (MNZ) enduro commissioner.
"Everyone seemed to enjoy the course. We had 112 riders enter and not one injury to report.
"The next round at Masterton may be a bit wet too, but that's to be expected in winter.
"Seth is certainly in the running to take the overall title this year, but a lot can still happen in the three rounds that remain."