A pink postcode refers to a suburb or city where a lot of gay people live, hence a 'gay' or 'pink' postcode. A pink postcode can also be referred to as a 'gay' or 'rainbow' postcode, but
'pink postcode' is more commonly used.
In Sydney, Australia, the well known pink postcodes (well suburbs) are Newtown, Glebe, Leichhardt, Annandale, Marrickville, Enmore, Dulwich Hill and Oxford Street of Sydney's CBD.
Tourist: I wanna go to Glebe to check out the new art gallery!
Australian: Don't go there, it's a pink postcode.