Melbourne Zoo went into lockdown on Monday after an orangutan escaped its enclosure.

Visitors were whisked into secure buildings, while the 11-year-old orangutan was on the loose in public areas for about an hour.

The Sumatran orangutan, named Malu, was eventually sedated with a tranquiliser, a zoo spokeswoman said.

"Melbourne Zoo staff acted quickly to lock down the zoo and all visitors have been safely ushered into secure areas," the spokeswoman said.