As per NOR

3.7 Each entered boat must have completed a qualifying race or cruise of not less than 250 nm on open water with the prospective skipper in charge.

Evidence can be:
Results from a yacht race you have completed – for example the 2014 Round North Island
Or a description of the journey

Between XX and XX the vessel XX completed a 250 nm passage with XX as the skipper. The course sailed was ( for example   Auckland to Mardsen Cove as part of the Route 66 race, (then after dropping crew  at marsden cove – or whatever), we continued immediately to XX, returning to Auckland XX time).

PLUS a log of the trip, which could be a GPS log or the course plotted on a chart.