Van der Poel was detained on the eve of Sunday's elite road race at the world championships in Wollongong, where he was among the favourites.
The Dutchman, who has won a stage on the Tour de France, was due in Sutherland Local Court on Tuesday but pleaded guilty on Monday and was fined A$1,500 (£910), according to a court statement.
He is reportedly on his way home to the Netherlands, with his lawyer Michael Rowe saying his passport had been returned and he was free to depart Australia.
Police alleged that a 27-year-old man had a verbal altercation with two girls aged 13 and 14 on Saturday evening and pushed them "with one falling to the ground and the other being pushed into a wall causing a minor graze to her elbow".
Van der Poel was charged with two counts of common assault. The court statement said he was fined A$1,000 (£606) for one charge and A$500 (£303) for the other.
"It's a sad result because I think obviously he was very frustrated with teenagers knocking on his door causing him to not be able to get to sleep," Rowe told Dutch broadcaster NOS outside the court.
"He's gone out to speak to them, they've knocked on his door a number of times... it was very plausible how it all happened and it's very disappointing that he's been convicted of this offence."
Rowe said he planned to appeal with Van der Poel having "a great reputation".
Let down
"He's flown all the way to Australia, it's a massive event, only those who understand cycling realise how big this event is," he added.
"He felt like he let his country down, he felt like he let his team down. It's a long way to come to have circumstances like this. He'll learn by the experience."
Van der Poel started the 266.9 kilometre (165.4 mile) race around the coastal city of Wollongong south of Sydney on Sunday, but retired in the early stages.
Belgian broadcaster Sporza said it spoke to the rider before the race began and he told them: "It's true, yes.
"There was a small dispute. It was about noisy neighbours and they are quite strict here."
Christoph Roodhooft, a director of his Team Alpecin-Deceuninck, said Van der Poel was shattered by the experience.
"He didn't sleep all night and was mentally a bit broken as well," he told reporters on Sunday.
"He was expecting a lot of this day (race) and did all he could in the last two months after his bad Tour de France. He had found joy and happiness again in cycling and was hoping to have a nice race."
In his absence, Belgium ace Remco Evenepoel stormed to victory with a spectacular solo ride, finishing two minutes and 21 seconds ahead of Frenchman Christophe Laporte after the six-hour and 16-minute endurance test to claim the rainbow jersey.
Van der Poel has enjoyed a strong season with wins at Dwars Door Vlaanderen and the Tour of Flanders, third at Milan-San Remo, fourth at Amstel Gold Race and ninth at Paris-Roubaix.
