A woman wanted for robbery and assault caught the attention of Sydney police when she was spotted littering in Campbelltown in Sydney's south-west.

Police say the 44-year-old woman threw a styrofoam cup on the ground.

When they went to reprimand her for it, the officers realised there was a warrant out for her arrest.

Inspector Bill Slatford alleged she had a key card and jewellery with her that belonged to a woman she had assaulted during the week.

"She was arrested, she was searched, and police found property which they believe was stolen," he said.

"Later enquiries [regarding] that property led to a lady at Rosemeadow.

"We ascertained that a robbery had occurred earlier in the week, which the lady that had been arrested was responsible for."