Chris Wurr has done some detective work in trying to establish when this photo was taken:

Further investigation reveals that this photo appears to have been taken between 1st June 1922 and 10th May 1930.
A2 837 was issued new to traffic (ex Newport Workshops) on 16th June 1910 as A2 No. 766. It was re-numbered 837 in December 1925 in the general re-numbering of that time.
On 16th April 1930, it was reclassed as A1 837. Later it was worked over in the shops with superheating and re-classed and re-numbered again and appeared as A2 838 !
It is shewn as scrapped on 3rd March 1951.

If we look closely at the picture (and there is a slightly sharper rendition of it in Rodney Hudson's "Locomotion A2" book), the station building on the Up platform has a signal bay and the Up Starting appears to be on its own post (not with the Down Distant on the back of it).
This dates the picture after 1/6/25 (ref: Jungways p136).
The number plate on the loco is of a newer style than the original, (see the A2 plate) and the loco is in virtually original "as built" condition (except the compressor has been moved forward from the original location on the Fireman's of the lower firebox) which indicates a simple re-numbering.
Had the number plate been of the original seriffed ornate style, it would indicate that it was the original A2 837 issued to traffic on 28th Feb 1913.
However this 837 was superheated on 22nd December 1920 and thus, its front end would look somewhat different to the one in this photo. Not only that, but the time frame would not tally with the concurrent signalling arrangements at Gisborne, as depicted in the photo.
BTW: the original A2 837 was re-numbered 908 in August 1925.

Photo circa 1930 from the Frank Kelly collection.