Hi Dave,

The 'jack' is the NZ national flag for that period, had been adopted in 1921. If the ship is underway there's no jack, and probably just the White Ensign in its sea position. If in action the NZ ships hoisted the national flag among the battle ensigns.

I don't recall if there's specific reference to these points, but Jack Harker wrote an interesting book (HMNZS Achilles), as well as others on the other RNZN (or almost RNZN) cruisers - Leander, Gambia and Neptune. Each is worth a read and help give better details about the ships and their conditions at specific points in time. The author had served aboard most of them.

Best wishes,

Simon