Bay of Plenty's Cody Cooper has one hand on the 2017 New Zealand Motocross Championships trophy and won't be letting go any time soon.

The reigning five-time MX1 champion extended his lead to 13 points over great rival and last season's runner-up, Australian Dean Ferris after the second of four rounds at Rotorua on Sunday.

The 33-year-old Mount Maunganui rider took command of the championship during the opening round of his title defence in Timaru a fortnight ago.

While weather conditions improved following the rain-delayed start to the day in Rotorua, the track remained tough and extremely deep ruts made racing difficult and passing almost impossible.

But the Honda Racing Team rider had all the answers for that.

Cooper twice finished runner-up in the early part of Sunday, behind Yamaha's Ferris and then another Australian visitor, KTM's Luke Styke, before letting rip in the third and final race of the day, with Cooper leading from start to finish to cap off a brilliant weekend.

In both the day's earlier races, Cooper had to call upon all his vast repertoire of race skills – the same ones that have made him champion in New Zealand for the past four consecutive seasons – to fight his way through traffic on a track that offered only a few good passing options.

Struggling at the start of the day, Cooper confesses he felt unbalanced on the bike.

'But then we adjusted the positions of the bike's front forks and I was right after that. I actually used a sand tyre in the last race and it worked brilliantly, even on the hard-pack sections.”

'The track was unreal, but very hard to pass on. When I got a bad start I had to ride aggressively to push through traffic and I think I hit a few people today. But that's what you have to do sometimes.”

Cooper adds it's fantastic he was able to leave Rotorua with a bigger points advantage than he started with.

'I'm riding within my comfort zone and feel that, if Dean (Ferris) steps it up, I can still go harder.”

Just as had happened at Timaru, Ferris was forced to settle for runner-up spot again at Rotorua, while it was another Honda rider Todd Waters, also from Australia, who claimed the final step on the podium and moved himself from fourth to third in the championship chase.

The 2017 New Zealand Motocross Championships series heads to Palmerston North for round three on March 12, with the fourth and final round set for Taupo on March 25.