New Zealand Law
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There is no law which says that you have to keep using any particular surname. So even if you have been married you can continue to use the married name, and you can revert back to the previous name at any time. There is no law which says that you have to change your name when you get married, and so some continue to use their maiden name. So someone can easily change their name back to their maiden name just by using the name, although generally you do need to take some more formal steps with some documents such as the passport. If you want to check whether she is married or not, you would need to check with the registry where she got married, whether that was in the United Kingdom or New Zealand. That is not an open registry to search so you would need to contact the country where she got married and check the information from their Births Deaths and Marriages departments. I don't have access to that information as you generally have to pay a fee for the search.
Its pretty easy to get divorced in New Zealand, and I think also in the UK. But certainly it does seem as though she may not have done so yet,