Hi - they are unusual speeds in that they are very close - are you referring to Wi-Fi connections from your device to your Optus NBN/ADSL modem or are you using an Optus Mobile Broadband Modem for connection to the Internet? What is the equipment model number of the modem and your wireless device?
Is your device set to the 2.4 Ghz or the 5GGhz SSID Broadcast name of your Optus modem's Wi-Fi home network?
Some of the early version F@ST3864 modems were single band 802.11n only, if yours does have AC NBN, V3 AC or V3HP after the 3864, it is a single Wi-Fi band modem. From a PC which is within 10 metres of the modem, can you look at your Adaptor Properties.
Windows - Control Panel, Network& Sharing Center, Change Wi-Fi Adapter Settings, select Connections - that will show you the transmission speed of your PC to the modem - if that is showing, for example a figure between 13 Mbps to 130 Mbps, the problem may be associated with your network connection - are you on ADSL or the NBN?
Mac - click the Wi-Fi Icon to see your transmission Connection speed to the modem.
Move around the room and do a few checks on your transmission speed to the modem. Once you have established a high rate, then do a speedtest.net test for Download and Upload. If your network speedtest results are still miserable, suggest you report the problem to Optus for resolution - they may supply you with a replacement modem or fix a problem in their network or pass it on to the NBN depending on their diagnosis.
More modern F@ST 802.11ac modems (2.4Ghz and the faster 5Ghz band) are also available on Gumtree for under $50, sometimes new, unused or only used for a few months including the current F@ST5366 model, so if it was a network problem that is resolved, you may get improved in home connection speeds with a newer model modem.