That is an odd one. If you filled in your location as Australia, it would have helped a bit.
I've seen many herons in the US, but none exactly like that. The closes I can think of is our Little Blue Heron. It has a white phase and it then turns blue. Maybe this one is almost done changing to blue but isn't done yet? Just a wild guess.
Thank you for the help.
The bird's picture was taken in Wollongong, a coastal city in New South Wales (East Coast). About 80km south of Sydney.
I saw this bird quite often even in city's parks. I also did saw a similar bird but in all white colour. I'll see if I can find the picture of the white's bird.