There can be 2 reasons to use an RCD in your case (in a German application):
Mandatory: Grid is TT or earth short circuit is too low to switch device off after 5 seconds (long cable?).
Other: You will be able to detect faults of the pump earlier which usually consist in tightness problems and can cause earth leakages.
Looking at the actual price of an RCD from about 25 Euros (some 50 Kiwi dollars?) we'd always do it with here.
