I've finally had a chance to test this - I had to swap back to the Optus modem as was using a Nighthawk modem. The speed here says 50,793 kbps. What does this represent? Is this the line limitation or have Optus not enabled a faster download speed?
For FTTN, Optus only provide three NBN speed tiers. 12Mbps, 25Mbps and 100Mbps.
You recorded ~50Mbps. This top speed would be because its the maximum speed your NBN FTTN wires can support. This may change (for the better) in the future as the wires may be upgradted but for the time being that seems likely to be your top speed. Maybe do a test in the morning (off peak) to confirm.
You should run some more speed tests in peak hour (~8pm) to see if congestion in your area is a factor (if it is your speeds will drop signifcantly below 50Mbps at this time)
Unfortunately 50Mbps is "inbetween" plans at Optus. Most providers sell a 50Mbps tier (perfect for you) but you'll need to decide between the 25Mbps plan $80 (all the time) or the 10Mbps plan $100 (only does 50Mbps max)
PS. Optus will be in touch with 8000 fttn customers before the end of March who were sold 100 Mbps plans they couldn't actually get. You're probably on the list. Not sure what options or refund you'll be offered.
Although two tests is hardly representative, you don't say what modem Nighthawk you have. But even the cheapest seems to be worth at least two times what the Optus modem sells for. It wouldn't surprise me to see a %10-%20 improvement in handling of FTTN signals.