Which of the multi-technology NBN’s do you have? Some of them are physically incapable in providing 50/20 to some premises. Is this your historical performance tested over multiple times on multiple days, or once off recent tests?
Since you are in Nimbin NSW (is there another?) it is curious a speed test is running to a node in Sydney, so the results will not be optimal. It seems you should be connecting to a server in Brisvegas, not Sydney metro, does it not? Is the Sydney node in the speed test one you selected or one speedtest automatically picked? It could have something to do with Exetel, but.
Have you lodged a formal complaint with Exetel for an investigation. If your NBN is FTTN and you are a distance from the node, your best possible speeds might be around 25 mbps (similar to ADSL2+ best case), which seems what you are getting. Some other NBN technologies could be similarly inferior. If that is the case they should not have sold you nor charged you for a 50/20 service.
Last year the ACCC came down on RSPs for selling products that physically could not be supplied to specific premises. You might be one. This page may be good information for you.
FWIW VPNs degrade performance at the best of times, often significantly, so your comparative speeds with and without the NBN are meaningless. The loss is in the VPN server itself, not in the NBN or Exetel.
