If you're not ready for the skills test you can do more hours without paying, is that right?
Nope, if you need to repeat a flight or two then (at least when I was in NZ) then that might be considered as a blip and won't be charged.
If you struggle and need to repeat syllabus flights then expect to be charged $$$$. I was fortunate and never had to repeat training flights but a few on my course did and ran up rather large bills which had to be cleared before moving on to the next stage of the training.
Other reasonable additional costs include transportation in NZ and the CAA licence issue fee as mentioned in earlier posts.