Just confirming that marriage record (NZSG index) shows bride's surname as
"EGAN". [She doesn't appear to have been born in New Zealand.]
Your man appears to have used the names "David Richard John" (sometimes shown as "David Richard J." ) in electoral listings.
This looks to be his first listing in 1938.
1938 - Wellington - MastertonALLAN - David Richard John - Branscombe Stn. (Station) - farm handGlad you've managed to locate him after so long a search. Good work on your part in connecting "all the dots".

~ Lu