Nicobar Pigeon.

How common are these in Australia...They are labelled in a few of the aviaries at Melbourne Zoo but I am yet to see one.

common enough go for around $4000 a pair

Taronga has several.

http://www.zoochat.com/34/nicobar-pigeon-29154/
http://www.zoochat.com/34/nicobar-pigeon-89058/
http://www.zoochat.com/34/nicobar-pigeon-89059/
http://www.zoochat.com/34/nicobar-pigeon-head-74222/

There are currently 62 (on isis) in Australian zoos, 1 male at melbourne zoo, 3 males and 4 females at perth zoo, 2 males, 6 females and 8 unknowns at adelaide zoo and 18 males, 17 females and 3 unknown sex at taronga. I am unsure of how many are kept privately around australia but with a price of $4000 a pair there wouldn't be too many.

There are quite a few in private hands.
I would say atleast a number that equals the zoos