@Chuan16 I can understand your guests' reasoning here, but I don't think this is a good idea for either you or the guests.
If during the 2 months of storage there was something such as a natural disaster, a burglary, a flood, or an issue with mold or pests that caused their property to become damaged or go missing, you don't want to be left with the liability for it (as surely you'd have enough on your plate in those circumstances already). Also, you don't know how your experience with the guests will go, and no way to be sure you'll find them trustworthy.
Luckily, you're in New Zealand, where there's no shortage of businesses catering to backpackers just like your guests. Major hostels offer multi-day backpack storage options (though not all of them are secure), and long-term storage facilities tend to offer monthly rates on secure lockers around 1 cubic meter - which shouldn't be a drain on the budget for people who can afford to fly to NZ. These businesses are heavily insured, and much more prepared to compensate the guests if anything happens to their stuff. As a host, you're much better off just sticking to the services that are offered in your listing and referring your guests elsewhere for any additional needs.