My civil law partner will be working in UK for a year. She has a UK passport. I have a New Zealand passport. Will I be able to stay in UK beyond the 6 month limit?
Yes, but you'll likely need to get some kind of visa to do so.
Apparently there is a rough rule of thumb of "6 months in every 12" that you can stay on a tourist visa (see answer here) - so just leaving and re-entering to immediately get another 6 months may be risky. If you are denied then it goes to a permanent record and you are more likely to be denied again in future. And, if you are like most kiwis in the UK, you will want to make the most of being close to Europe and travel around a lot, so being worried every time you leave the country is not a great option. You also haven't mentioned if you want to work during that time, not an option on the 6 month tourist visa.
You can get a "family visa" to join/remain with your partner (depending on if you are already in the UK). They require proof of relationship and some financial evidence (of "£18,600 per year if you’re applying only for yourself") as well. See the documentation here for more details: https://www.gov.uk/browse/visas-immigration/family-visas