I'm not aware of a return ticket to anywhere in the world you can buy in advance without atleast stating your outward journey date...
If any airline was to offer such a ticket, I would expect it to be be more expensive than their most expensive peak season tickets, as they will be providing you with flexiability to travel as and when you need.
I would just keep the money in a savings account so it is available to spend if/when the time comes you need to make emergency travel to NZ. Some airlines offer bereavement fares, but these are becoming more and more harder to find as airlines withdraw them.