No.
Am not sure where you are getting the information from re the number of miles you need to fly to SYD and back and only have to pay the 'taxes' from but 31k simply won't cut it (plus award tickets to SYD are tricky to get and you would need to be flexible on dates)
To get a 'free' Econ ticket to SYD you need 100k miles each.
And as for the taxes the biggest element of those is the VS fuel surcharge which is not a tax (even though they try to pass it off as one).