If the aggregator website doesn’t provide the name of the rental vehicle company or vehicle model (as is often the case), it might help to copy the description of the vehicle into your search engine, as rental vehicle companies often just copy snippets from their own website into their aggregator listings.
For relocation deals, rental vehicle companies stipulate a maximum kilometre amount, with any kilometre driven above that being charged at a fixed per kilometre rate.
In this example, 3,000km are included, which is plenty to get you from Auckland to Christchurch with heaps of side trips along the way.
In this example (and in fact in most relocation deals that are not super restrictive) the ferry from the North to the South Island is not included.