In the Practical Guidebook, which is provided on collection of your vehicle, you will find the procedure, price and contact details for extending your lease whilst in Europe. (This cannot be done through your travel agent in New Zealand.)

The prices to extend your vehicle whilst in Europe, in 2013, are as follows: 

208, 207SW, 207CC	EUR25 per day
308, 308SW, 308CC	EUR27 per day
3008, 5008	EUR30 per day
508, 508SW	EUR33 per day
Partner Tepee	EUR27 per day
RCZ, 4008	EUR33 per day
807, Expert Tepee	EUR36 per day

Note: if you extend your lease, then change your mind and return your vehicle early, you will not be able to claim a refund for these unused days.