If the fares permit refunds, then the refund will be the difference between the fare paid and the one way fare - you cannot circumvent the one way fare by getting a return ticket and cancelling

If you want a cheap ticket, perhaps try another airline

There seem to be fares with China Airways at €2267 , Turkish and Singapoe at €3100 to WLG

To Auckland there are fares starting at €2036 with Thai and €2408 with Cathay Pacific - a flight from AKL-WLG will not cost very much