Not sure where in the RAF WA100 - WZ999 you are looking? My issue is from 1983.
My booklet says WE409 to RNZAF 21.3.57 WE420 to RNZAF 3.12.56
It states nothing with regard to their final ends in NZ service
The delivery date you quote for WE420 was to RAF and not to NZ From an Australian site listing also NZ aircraft, no transfer date is given for any of the above, so the dates by Jim could well be correct.
With regard to WE409 the same site says it was with 14 RNZAF in May 1957, which fits quite nicely with Jim's info.