A Sydney council has been forced to install internet search filters across its library network after a man was caught by school students looking at porn on library computers.

Freshwater resident Margot, who didn't want her last name published, said her 16-year-old daughter was studying at Manly Library with a friend after school last week when they saw the man looking at pornography on a computer in the middle of the library.

School students filmed a man looking at porn at Manly Library.

"They were sitting at a table and a guy behind them was watching fairly explicit porn," Margot said.

"He was in close proximity to them and a woman sitting next to him moved."