I question your estimates.
My personal experience flying a >5700kg jet is that I very rarely burned more than the planned burn, let alone using all of the variable. I suspect also that most minimum fuel plans would include an alternate and given that it is relatively rare to divert in Australia, a minimum fuel plan is unlikely to result in using FIXED reserve. Nice day, no alternate, plans would typically have a company fuel reserve that is more than fixed plus variable. Where I used to work we always planned for at least an hour overhead.
Finally, you don’t suddenly discover that you’ve eaten all of your variable after you get to your destination, you will be running an en route fuel check and note that the fuel situation is trending downward well in advance. This would be the time to divert for a top up.