When did the Northern Territory become a colony of Australia?

The Northern Territory was never a colony of Australia.

The Northern Territory was founded under the name of "Northern Territory" in 1911, which is when it was separated from South Australia and transferred to Commonwealth control.

From 1825 to 1863, the Northern Territory was part of New South Wales, and from 1863 to 1911 it was part of South Australia, at neither time being a separate territory or colony - though there was the outpost of Port Essington on the northern coast.

On 1 January 1911, the Northern Territory was removed from South Australia's rule and transferred to Commonwealth control.