Motocross riders battle mud and rain in Appin: Photos

"The Appin track was borderline dangerous by the end of the day."

That's the view of MX Nationals series organiser Kevin Williams after Appin hosted round one of the 2019 championships on Sunday.

The Macarthur Motorcycle Complex in Appin resembled a mud pit for the competition, while riders battled against heavy rain and windy conditions.

Williams said riders faced "treacherous" conditions for riding on Sunday.

"It was the most difficult track we've had in 23 years of running this competition," he said.

"Riders were getting stuck in the mud. We had to get a bobcat at one stage to help out.

"It was the toughest conditions in Australian motocross."

Williams said more than 800 people gathered on Sunday to watch the motocross action.

"We normally get up to 4000 people, but there were some die-hard fans who braved the treacherous conditions," he said.

Three classes were run to open the Pirelli MX Nationals championship.

Huskvarna rider Todd Waters arguably handled the conditions best, winning the MX1 category.

KTM's Hayden Ballas finished runner-up and Yamaha rider Kurt Gibbs took third place.

KTM rider Jye Roberts won the MX2 class from Yamaha duo Jay Wilson and Wilson Todd.

Yamaha rider Maximus Purvis took out the MX Development class from Honda's Rhys Budd and KTM's Michael Murphy. 

Williams said one round of each class was run before the competition was called off early due to the weather.

Two rounds in each category were scheduled to be held on Sunday.

"We saw more than 100 millimetres of rain fall while we were there," he said.

"Otherwise, it was a really good day."