The Tour de France stage winner was arrested hours before Sunday's men's world championship road race in Wollongong where the 27-year-old was one of the title favourites.
He was due to appear in Sutherland Local Court on Tuesday but made an early appearance on Monday to plead guilty.
Magistrate Hugh Donnelly convicted the two-time Tour of Flanders champion and ordered he pay fines of $1000 and $500 for the two assaults.
Police had alleged the cyclist pushed two girls in the hallway of his Brighton-Le-Sands hotel, with one falling over and the other hitting a wall, suffering a minor graze to her elbow.
"I went to bed early and many children in the hallway of my room found it necessary to knock on the door continuously," the rider told the Belgian website Sporza before the start of the race.
"After a few times I was done with it. I didn't ask so nicely to stop. Then the police were called and I was taken.
"I wasn't back in my room until 4 o'clock. That's certainly not ideal. It's a disaster, but I can't change anything anymore."
The 27-year-old started the 266.9km race on Sunday but withdrew about an hour into the event.
Dutch media reported van der Poel's plea was entered to allow him to travel back to Europe on Monday and that he will appeal the conviction.
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