Hi Jacky
Just spotted your incoming post.

To ease the confusion regarding the Army records at Archives -Wellington ... it's probably best to just follow the instruction given in Beg's post, and add a request for digistisation of Leslie Benjamin's file at the link provided.

[Even I have difficulty at times comprehending what is written in Archives NZ's blurb.

But ALL WW1 files now have the word "restricted" added to them ... and it's not made easy by Archives' to navigate their website and find the ordering page or further instructions and explanations. ]
Once requested though, the Army record should appear online at the Archway (website) within two weeks.
I remain confident that "Benjamin Leeson" is indeed Leslie Benjamin.

(Possibly a variation in name has come about, so as to distinguish him from his father ?)
I haven't found any (further) records for a "Benjamin Leeson" ... and there is just the one death of a "Benjamin O'NEILL" (in 1927) showing on the NZ index.
Fingers crossed then that the NZ Army record provides the confirmation you're needing.
{I know I said earlier I'd add some further info for these O'NEILLs - will wait though till army info is to hand.}
Cheers
~ Lu