Police trying to rescue a victim of an alleged paedophile in Sydney's west have instead ended up arresting a 51-year-old man.

The rescue was launched yesterday after police in Victoria arrested a man and allegedly discovered an internet chat history with a 14-year-old girl who had agreed to meet him.

Inspector Simon Maund says officers went to what they thought was the girl's Penrith home, but instead found the 51-year-old man.

"The officers conducted a search of the premises and a computer was checked. Investigators will allege that it contained child pornography and the computer was seized," Inspector Maund said.

"Officers were able to determine that the 'teenager' was a fictional internet alias."

The man has been charged with possessing and disseminating the material, and has been kept in custody to face court today.

His alleged chat partner is facing child pornography charges in Victoria.