Question: Can I extend my visa to stay since it is about to expire and there are border restrictions?

Answer passage (source: Australia):
page_title: Frequently Asked Questions
section_headers: Employer sponsored temporary work visas
What can I do if I cannot return to my home country?
section_content: If you cannot return to your home country, you need to maintain a valid visa
and engage with the health system when necessary.
Where you have been stood down by your employer or had your hours reduced but
remain employed, you can access up to $10,000 of your Australian
superannuation tax-free in the 2019-20 financial year.
For information on extending a stay in Australia if you are unable to depart,
see Staying in Australia.

Reviewer feedback on this answer:
There could be more information, but the info here might be helpful.  It does talk about getting a stay extended.
This answer does not relate with how to extend your visa, but does relate to maintaining a visa.

Reviewer ratings:
Acceptable
Could be Improved

Answer passage (source: Australia):
page_title: Frequently Asked Questions
section_headers: Temporary visa measures supporting the agriculture sector
Working Holiday Maker visa holders
Due to COVID-19, I couldn’t complete the specified work I need to apply for my second or third WHM visa. Can I extend my visa to give me more time to look for work?
section_content: If you have not completed the three or six months of specified work required
to apply for a second or third WHM visa, you should apply for another visa to
remain lawfully in Australia until you are able to depart.
If you are working in the agricultural sector and are unable to return to your
home country, you can apply for the Temporary Activity (subclass 408)
Australian Government Endorsed Event (AGEE) stream visa. This also applies to
those working in other critical sectors such as food processing, health care,
aged care, disability care and child care.
This visa will allow you to remain lawfully in Australia, and continue
working, should you wish to do so, until you can return to your home country.
If you are applying for the visa to work or continue working in a critical
sector you must have evidence from your employer that you have ongoing work
that an Australian citizen or permanent resident cannot do.
Work undertaken on a subclass 408 visa does not count as specified work for
the purpose of applying for a second or third WHM visa.

Reviewer feedback on this answer:
This answer does not relate to extending a visa, but having to apply for a new visa.
This answer talks about using a visa, but it doesn't say anything about extending a stay using a visa.

Reviewer ratings:
Bad
Could be Improved

Answer passage (source: Australia):
page_title: Staying in Australia
section_headers: I am a Seasonal worker or part of the Pacific Labour Scheme
Subclass 403 visa holders in the Seasonal Worker Program
section_content: Your visa cannot be extended. However, the Australian Government has
introduced new measures for those working in critical sectors in response to
COVID-19.
You may be eligible for a Temporary Activity (subclass 408 Australian
Government Endorsed Event (AGEE)
stream)
visa during the COVID-19 pandemic, if:

your visa is about to expire
your visa has already expired
you can’t leave Australia
you are working in a critical sector, like health, aged care or agriculture.

This visa will allow you to stay lawfully in Australia, and continue working,
if you wish to do so, or until you can return to your home country. Seasonal
Worker Programme and Pacific Labour Scheme arrangements, including those
relating to employers, will be carried over to the new visa arrangements,
continuing the link with the agricultural sector.

Reviewer feedback on this answer:
This answer relates to the question and gives information in regards to extending a visa.
This is a good answer.  It directly addresses the issue about extending a visa and states that it can't be done.

Reviewer ratings:
Excellent
Excellent