There are two other antennas that I occasionally use on the R6.
1.) Watson WSMA-801 - approximately 9 inches long with most of bulk near the base and is slimmer than the stock which is 7 inches long, It sells for approximately $35 USD. It increases the "reach" by probably 50%.
2.) Diamond SRH77CA - approximately 16 inches long with most of the bulk near the base and is slimmer than the stock antenna. It sells for approximately $25 USD. It increases the "reach" by probably 75%.
However, couple of thoughts...
The R6 can easily front end overload on strong signals especially if they are close by. My house is 10 miles from the Lexington Airport. With the stock antenna, I can hear the tower and approach/departure controllers at a "2" bars out of 7 or 8 bars on display, but the planes are always 8 or more. HOWEVER, if I am at the airport using the R6 with stock antenna to pick up someone and sitting in the "Cellphone Wait Area", I get front end overload on all of the controllers which is very noticeable however the planes are fine. The tower is only 500 feet or so from the Cellphone Wait Area. I can offset that by laying the receiver flat on a seat, but anything near the window, I get the front end overload. However, if I am at the other end of the airport (say 1/2 mile away), I get no overload on the R6 with the stock antenna. With either of the other antennas, I get overload at both the Cellphone Wait Area as well as 1/2 mile away. At home with the other antennas, I usually get 3 to 5 bars out of the 8 when the controllers are talking.
If I have the 16 inch antenna on the R6, it is very top heavy and almost defeats the purpose of having a small receiver. The 9 inch Watson antenna is about the same as the stock antenna since the bulk of its weigh is near the attachment point to the receiver.
BTW, I know New Zealand is metric but I am not good at converting numbers.
Don't know if either of these antenna are available locally to you. Shipping cost from the US is probably greater than the cost of the antenna.
EDIT: It looks like according to the manual that there are 11 bars on the strength display. I get all 11 most of the time for planes within 25 miles of me with the stock antenna. If the planes are on Indianapolis Control Center and high altitude, it is probably 100 miles with still reasonable sounds.