Vouchers refund frustration

Scribber99

I’ve  seen a few posts on this theme but nothing that deals with my problem specifically. We just flew back from Australia ten days early and I’m hopeful of recovering the additional flight costs from my insurer. Most of the activities we had booked in Sydney for this week have refunded immediately however the Sydney Bridge Climb aren’t refunding, but have offered me a voucher valid for three years. Whilst this would be fair enough for an Australian resident, the chances I could afford another holiday in Australia are vanishingly small (especially now!) and so this voucher is completely useless to me. My insurer says I have to try to reclaim the £325 from my credit card, but MasterCard tell me that because I’ve been offered a voucher I’m not covered by them as the service is still technically available to me.
The irony is that Sydney Bridge Climb’s usual T&Cs don’t offer refunds or vouchers for cancellations and they think they are being generous by offering the voucher, whereas what they have effectively done is lock an foreign visitors out of being able to claim on their insurance.
can anyone at MSE throw some light on this? Is MasterCard right in what they say, given that, short of a lottery win, here’s no way I could feasibly use this voucher,  do I have any rights under either British law or Australian consumer law to recoup the money? Is anyone else going through anything similar.
Any advice would be greatly appreciated.