Qantas tax refund adjustments

I booked reward flights from Brisbane to Alice Springs via Sydney. I paid $188 in taxes for this booking. Qantas later cancelled these flights and rebooked me on the direct flight from Brisbane to Alice Springs on the same day. Taxes for this direct flight are $57. Qantas did not make any adjustment for taxes. I phoned Qantas and they have agreed to refund me $131 but have been told it will take 4-6 weeks.

Does anyone know of Qantas’ policy regarding tax adjustment refunds and why this does not happen automatically?

Cheers