The cheapest fares are usually based on return travel. "Round World" tickets will only be issued if they take you back to the point of origin. Some of these tickets will not issue "open" dated sectors, but it is easy to put in a dummy date and change this later when you know when you want to travel (but there may be a change fee from nothing to hundreds).

A simple round world ticket may be from your Europe start point via NRT to SYD with a return to Europe via SFO. I have used such tickets and LH issues lots of them in Australia. But, LH may not be a cheap issue in Europe.

Make sure know the immigration rules for your planned stay in Australia and NZ. Your visa may stipulate a maximum stay and immigration officials may want to see evidence of your planned exit from the countries. Therefore declaring you wish to stay for 6 to 8 months without evidence of return travel may be problematic.

You might find individual fares in Australia and across to NZ may be cheaper than a Pass ticket. The Low Cost Carriers are Tiger, jetStar and Virgin Australia. Qantas and AirNZ can be cheap sometimes. Their respective web sites make it easy to get ideas of pricing.