Beautiful city, beautiful country! Maybe a machine shop might have a way to mount your frame in a bridgeport milling machine and ream it out? The cut is critical so that the top and bottom faces of the head tube will be parallel and that the headset cups will fully seat. A lot of elbow grease with a flat file and careful meauring with verniers might get you close enough in a pinch. If the cups will not fully seat you may be able to dremel out the inside of the headtube but the best tool for the job is a reamer.
Short story: Wife and i went to NZ for our honeymoon back in September of 2000. What a spectacular corner of the world you live in. We had an incredible adventure for 3-1/2 weeks; wine tour, black water rafting, bungee, jet boat, snow cat skiing (Mt. Potts), Rainbow ski area, hot springs, glacier visit, 4 days in Christchurch, lemon & lager (still drink the brew this way), incredible sea food and the friendliest people you will meet, etc. etc. When we came home we had no jobs, no apartment and $50.00 in the bank. I only 1/2 joke that we could have stayed another day with $50.00! My wife may take advantage of an opportunity to visit again this spring with my mom if we can pull it off. It would be a long flight with a baby in tow and i would stay home with the 3 & 5 yr old but this is an opportunity that doesn't come often (free lodging and car use).