1.: A UK and Australian slang term referring to paying for something in instalments (i.e. instalment payments, hire-purchase system, easy monthly payments). For example, 'to by something on the never-never'.
2.: Australian slang for the Outback. The never ending landscape; flat, sparse land as far as the eye can see. Might also be used for other sparsely inhabited desert country or remote and isolated regions in other parts of the world.
3.: Used in the term never-never land, as used in James M. Barrie's Peter Pan. A magical, dreamlike land. In Peter Pan this land specifically had the property that its inhabitants never aged. Other magical lands might be referred to as a never-never land for other reasons, but the term does imply a sort of timelessness.