New Zealand dependency comprising the southern and (from 1901) northern Cook Islands, annexed by proclamation on 11 June 1901 (transferred from the short-lived local Cook Islands Federation, already in the atlas as 'cook-islands-federation', whose own fate text names this exact transfer). Administered by New Zealand's Department of Island Territories via a Resident Commissioner, with a local Island Councils system; the Cook Islands Amendment Act 1946 introduced a partly-elected Legislative Council.
Achieved full internal self-government in free association with New Zealand on 4 August 1965 under the Cook Islands Constitution Act 1964, becoming the polity already in the atlas as the modern 'Cook Islands' record (Wikidata Q26988), which the atlas dates from 1965.
Standard scholarly history already cited (and OL-verified) in the atlas's own 'cook-islands-federation' record; covers first contact through the Kingdom of Rarotonga, the Federation, and directly into the New Zealand annexation/administration period this record covers.
Open Library-verified (work OL44964457W, published 1950 by the actual administering department mid-way through this exact administrative period) -- a contemporaneous primary-adjacent source describing the NZ Territory administration directly.