Query on condition 8858 on visa class 600

[KT2020]

Hi, my mother is on visitor visa in Australia (sub class 600). It was granted on 18 Feb 2019 and it is valid till 18 Feb 2022. Her visa conditions are that she can stay maximum 12 months in 18 months (condition 8858). She first came to Australia on 1 Jun 2019 and stayed till 25 August 2019. She was away for like 6 months. She came back to Australia on 10 Feb 2020, flew back to Singapore for holiday on 19 February 2020. She then back to Australia again on 24 February 2020. It was supposed to be 1.5 months stay and she was to fly back to USA. But with this corona situation, she is stuck here. We are confused on till when will she be allowed to stay. She is looking to fly back in December 2020 if everything looks good and she can get a ticket. Will she be allowed to stay till 18 Feb 2022 if she needs to stay a bit longer to get the right flight ticket to USA? She has pre-existing medical condition, so we don’t want to take any risk. She currently has medical insurance for her stay in Australia.