What NBN technology are you on? FTTN? Check here
The speeds you are getting are likely two main reasons.
1 . There is a fault on the NBN line (Contact Optus directly and they will get the NBN to look into it)
2. You are on FTTN and the top possible speed to your house is 30Mbps. (Contact Optus directly and ask them if you are FTTN and what your max line speed is.
Either way, if you've been paying for 100 and getting 30Mbps then Optus owe you a refund of $30 for every month you've been having issues. Request that as well.
PS Do confirm you can get 27Mbps. That speed indicates it is not an Optus setting that is limiting you to the usual 25Mbps speed tier.