An intruder broke into a naval base in Darwin, Australia, allegedly assaulted a crew member on a moored patrol boat, and stole a number of weapons, local media reported on Friday.

A big search is under way after the dramatic security breach at the base in the early hours of Friday.

Police said the intruder overwhelmed the crew member, and removed a number of weapons from the vessel's armoury.

An investigation has been launched into the incident.

"Well, that is (security breach) something that we need to obviously look into very carefully. There is, as I said, an investigation under way from a police perspective, but I have also initiated a review of security on board our ships and establishments," Royal Australian Navy chief Vice Admiral Ray Griggs told reporters in Sydney.

The base has been locked down and police have been searching all vehicles leaving the area, police said.