An Electronic Travel Authority (subclass 601) visa permits you to engage in tourist or business visitor activities while in Australia. If you intend to travel to Australia for activities other than tourist or business visitor activities, then your ETA is liable for cancellation under paragraph 2.43(ea) of the Migration Regulations 1994. This paragraph provides (emphasis added) that a visa can be cancelled if:
in the case of a Subclass 601 (Electronic Travel Authority) visa—that, despite the grant of the visa, the Minister is satisfied that the visa holder:
(i) did not have, at the time of the grant of the visa, an intention only to stay in, or visit, Australia temporarily for the tourism or business purposes for which the visa was granted; or
(ii) has ceased to have that intention;
If your intention to travel to Australia is to seek medical treatment, then you should consider a Medical Treatment (subclass 602) visa.
Credit:stackoverflow.com‘