That's because a functioning auxiliary engine is mandatory passing Green Cape (underlining added by me):
10. MANDATORY REPORTING FROM GREEN CAPE
10.1 On approaching 37° 15’ South a boat shall assess and be satisfied that:
(a) Its HF Radio is fully operational and fit for purpose;
(b) The required number of liferafts are on board;
(c) Its engine and batteries are operational;
(d) The boat and its crew are in a satisfactory condition to continue; and
(e) The skipper has comprehensively considered the most current weather forecasts and considers that the boat and crew are fully prepared for the conditions forecast, and make a report by radio to that effect.
If the boat does not make that report or cannot contact the required radio vessel or radio station the boat shall be recorded DNF without a hearing (amends RRS 63.1)