Kyle James Henk Daniels faces 21 charges of inappropriate conduct to female students under 10 while working as a part-time instructor at a Mosman swim school during university.

"The crown case is that the accused has a sexual interest in young girls and that he acted on that by touching the vaginas of nine young girls," prosecutor Tony McCarthy told a District Court jury on Monday.

The 23-year old, who has pleaded not guilty to the charges, is facing a retrial in Sydney's Downing Centre over the alleged offences including touching girls on the outside of their swimming costumes to penetrating their vaginas on five separate occasion with his fingers.

The alleged offences occurred while the girls were being instructed in or around the pool, the jury heard.

Two complaints were made to the swim school, one in July 2018 and the other in February 2019. A formal complaint to the Department of Family and Community Services was lodged after the second incident and NSW Police were notified.

Police then descended on Daniels' home on March 12, 2019 in a widely publicised arrest.

The following day, the swim school emailed parents of its students notifying them of the arrest and the charges. The email prompted other parents to come forward with complaints about Daniels.

"It felt like a worm was in her private part. His fingers went inside her private part. It felt a bit sore when he did it," Mr McCarthy told the jury, recounting the evidence one girl gave police.

Another girl said the swim coach had held her in an odd manner while she was practising freestyle in the pool.

"It felt really uncomfortable, weird and she didn't like it," Mr McCarthy said.

When first notified of the complaints, Daniels allegedly denied he did anything wrong, saying any touching could have been by accident. At that time in August 2018, he agreed to use a less hands-on teaching style during his classes, the court heard.

The trial before Judge Kara Shead continues.

Australian Associated Press