Changing retailer doesn't change where your electricity comes from. There are lots of different energy generators throughout New Zealand and your power could come from any of them.
If it helps, think of it as vegetable soup. All generated electricity, whether is be hydro, wind, coal or gas, is all fed into the national grid and mixed up. From there, electricity retailers buy it on the New Zealand Electricity Market (NZEM) and sell it to their customers. So there’s no way of knowing where your portion of the vegetable soup was generated or whether it's renewable or not.