Originally was up to the 25Mb/s paying for (according to Speedtest), but lately lucky to see 15Mb/s, at any time of day - mid morning, after lunch, or at night. Postcode 3095.
Sounds like NBN. What technology type do you have? And how are you testing? Over WiFi or plugged directly into the modem?
Have you run any troubleshooting steps yet or spoken with support?
NBN, HFC, wifi. ALL my testing has been done in the same setup, config. Nobody streaming, just me and Mrs browsing. Using Speedtest off google. bUsed to be consistently in the 20s, but lately its dropped another 5M.
What diagnostics can I do from inside home?
First thing I'd do is connect to the modem directly via ethernet to rule out any issues with the WiFi. Have you performed a reset of the modems as well? Modems being plural as if you have HFC NBN you should have an Optus box and NBN box. There should also be a reset button on the NBN box, make sure to reset it as well.
Outside of that, if there's no improvement you'd need to call tech support.