The New Year's Eve actor found more than 300 items to donate to the Children's Resource Network of the Central Coast charity.
Zac, 24, handed over the clothes last month to raise funds for the organisation, which helps local children in need with clothing and school supplies.
Charity boss Lisa Rey says she didn't recognise him at first but soon became starstruck when he said who he was.
She told Sanluisobispo.com: 'He looks at me and says, 'Hey, I'm Zac Efron'... And I screamed. I'm screaming like I'm a 13-year-old girl... His gift says a lot. It's not only the clothing. It's having something special that other kids wouldn't have access to. It shows them that people do care.'