it could go eatherway, page-6

Absolutely HTA have the right to question...
Optus used some of the same arguments as Telstra.

The decision has already really been made and HTA cannot legally query the decision.
HTA are questioning the decision making process, which is their best legal option.
If the decision making process is broken, then ultimately the decision may need to be reassessed.

From what I can see the ACCC were given a lot of information and have made an informed decision.

If the ACCC processes are fine, the decision will stand no matter how ridiculous their decision is deemed to be.