Undercover police detectives have charged a man over the alleged exploitation of a child.

Detectives posed as a 40-year-old relative of a nine-year-old girl and were in contact with the northern beaches man online.

Police say a 43-year-old man spoke about sexual acts he wanted to perform on the girl and acts he wanted the relative to perform.

The man was arrested outside a Collaroy Plateau business and charged with accessory before the fact of attempted sexual intercourse with a minor.

He has been refused bail to appear in court today.