I had the same issue as others have described, check the ethernet port light (depending on modem model I have the Arris CM8200).  The ethernet port light was flashing orange (100Mb port speed) not green (1000Mb port speed).  Because my router is a fair distance from my modem due to the location of my HFC/cable port I was using long (5 metres) Cat 6 cables (I tried 2 with the same result).  Once I moved the router and NBN modem closer together and used a 2 metre cable instead, voila 1000Mb port speed on the modem, and speed tests went from 100Mb/sec to around 250Mb/sec.   Swapped cables out to test and straight back down to 100Mb with port light changing back to orange.   So, try using a good quality shorter cable and see if that does anything.  If you have a different modem you can check the WAN port speed of your router to see if it's set at 100Mb or 1Gb per second. ... View more