In the States There is a product called "The Stick" ($139 US) http://faxswitch.com/stick.html that works very well with DSL and ADSL for sorting out your fax calls from your voice calls.
With the Stick your phones work line they did before you added the DSL because you only need one filter instead of having one for each extension. With a single phone line application with extension phones, a fax and a modem, you just plug everything in and your done. Your faxes automatically do to your fax machine or fax modem without ringing any phones and your voice calls ring all of your phones like normal. You can also dial into modems too. It is really very simple.
There is an installation diagram in the online manual under frequently asked questions about DSL. http://faxswitch.com/help/stick/stic...dsl_stick.html
The Stick works well with all phone company features except call forwarding (your fax calls get forwarded with your voice calls), phone company voice mail (since the stick answers all incoming calls you must use an answering machine because it can't send calls back to the phone company), and distinctive ring (the manufacturer has other products for that).
Since you live in New Zealand there will be an extra charge ($25) for the 220-240 electricity power requirements in your country. The company will add this charge to your order to include the proper power supply for New Zealand.
The RJ-11 phone jacks that you use in New Zealand are identical to the ones used in the States so you won't need any phone adaptors or do any wiring.
The only other thing that you need to be aware of is The Stick uses the US and Canada Bell Specification Ring back tone that the callers hear when they ring your phones so it would sound a little different to your callers than the normal double type ringback that I imagine your caller normally hear in New Zealand (it would sound like they are calling to the US). If you think this would bother your callers, it might be better to find a similar product manufactured in New Zealand. Otherwise I am confident that it will work correctly in your environment.
Besides this (the US ring back tone), the extra charges for the New Zealand Power Supply ($25 US) and the cost of shipping to New Zealand (about $70 US), I don't think you will find any other disadvantages of using this product.
If you can't find somthing locally, The Stick will solve your problem.