I have NBN 100 speed 4 but experienced less than 1 Mbps every night in last 10 days. Optus case management team can't help as they tested no problem during the day time, nor told me NBN technician don't needed. I'm very disappointed.
How are you measuring your speed?
What results does this speed test give you?
What kinds of speeds were you getting before the problem?
Are you doing the test on wifi or through Ethernet cable?
What type of NBN are you on FTTN ,FTTC, etc?