Are you saying that the speed was lower than before they were on HFC or slower than previously on the HFC?
At https://www.iinet.net.au/internet-products/broadband/vdsl2/ iiNet talk about the following contracts:
Fast: up to 30Mbps*, and
with the following small print caveat that "*VDSL2 Speeds: Actual throughput speeds may be slower and could vary due to various factors including interference, customer cabling and equipment, download source, and line length."
You are right that they seem to have covered their rear ends quite well. Depending on which your relatives were signed up to will determine the potential upper speed limits of downloads.
Please note that TPG owns the following are subsidiaries:
* Adam Internet
* Country NetLink
* Origin Internet
* PIPE Networks
and as has been discussed previously on this forum, TPG is not known for their customer focus. Since TPG bought iiNet in 2016, iiNet's customer service has diminished markedly.
As I'm sure you are aware if it is regularly & significantly lower than their plan advertised, your relatives will need to regularly measure and record their speeds to build a case under the Australian Consumer Law (ACL).
For more information on the ACL, please search the forum as there is lots of information on how to go about it.