Perhaps I don’t understand something - if your father is allowed in Australia for up to 3 months, what is preventing him taking the once a month flight that happens in the 3rd month of his visa?
Nothing technically, but you need to consider implications for the future.
The most immediate problem, is to get yourself a seat. For example, since he arrived at the beginning of Jan, he needs to be out of Australia by the beginning of April. The last CX flight to get him out legally would be
22 March. Any later, and he would be flying to Christmas Island instead. But what if this flight falls through because the pollies in Hong Kong need to continue with this blame shifting? My father would be straight to Christmas Island.
Secondly, if he just, I don't know, say, fly to Fiji then U turn back, he would technically be allowed back in, but it would be very obvious with what you are doing, and this could raise questions if he was to say, apply for a long term visa later.