Identification of an Australian pigeon in the NT
Your bird appears to be an Imperial Pigeon of the Ducula genus. Based on the limited angle of the lone picture of your specimen, my best guess as to the species is the Torresian Imperial Pigeon (Ducula spilorrhoa).

The species is a relatively large, pied species of pigeon found in various habitats (forest, woodland, savanna, mangrove and scrub) in Australia (including in the Northern Territory where you saw your specimen), New Guinea and nearby islands.
You can see more pictures, videos, and calls here, as well as additional info here.
Your specimen is unlikely the closely related  Pied Imperial Pigeon (Ducula bicolor) because:

D. bicolor is a  Southeast Asian species

D. bicolor has a bluish grey beak, whereas your specimen and D. spilorrhoa both have a yellow beak.
Note: Your specimen is also unlikely Ducula subflavescens because the feathers are white and not yellow, and it is unlikely Ducula luctuosa because that species is endemic to only a few islands.