Good milk shakes are found at some cafes. Not the thick shake variety in luminous colour. The Ritz Cafe, Adamstown is one source. Another is Jim's Dairy Delight and Milk Bar, Hamilton, years old, where the array of chocolates and sweets is much much smaller now and some unusual incidents involved the owner in the past. 

As an occasional drink, the milk shake is rather good, even better malted, but the quality varies. Sometimes they are lastly topped up with additional milk which defeats the purpose and ends up with a massive amount of drink. According to one cafe it is best to use day-old milk because it whips-up better. Skim milk is an option! And maybe malt is a wholesome addition.