A Sydney supermarket employee is at a loss to explain why he up-skirted customers and filmed his co-workers in the toilet, telling a court he could not stop.

Bibek Guragain was arrested in June after an 18-year-old woman noticed a phone near a toilet at Woolworths in Surry Hills.

The 23-year-old has since pleaded guilty to filming someone in a private act without consent and filming a person's private parts without consent, and says he still doesn't understand why he did it.

"I remember wishing that I could stop but I could not and I can't even explain it," Guragain said in a letter tendered in court.