Firstly the 457 visa system has been scrapped, so it is a little hard to apply for one (some applications are still in the pipeline).
Secondly the reciprocal health agreement we have with those countries is not Medicare and there are limitations on what is provided.
Thirdly, and the important bit is the requirement to be eligible is
to have applied for a Permanent Visa - the exception is Parent Visas.
There is no restriction on countries, you simply go to Medicare with receipt of payment for a
PR visa, your Medicare application form and the current visa you have to be legally in Australia such as a Visitor Visa or a Bridging Visa E. A short time later they will give you your Medicare Number and tell you your "Blue Interim Card" will turn up in the mail in around 2 weeks, you stay on the "blue card" until
PR is granted often a few years later. There is no difference in what is provided between the Blue and Green and I see Ukraine is not on that list!
https://www.humanservices.gov.au/ind...who-can-get-it
https://www.seekvisa.com.au/visas-el...-for-medicare/
This one also covers Temp visas and a statement of a time limit for Reciprocal Health cover.
https://www.immiaustralia.com.au/blo...-for-medicare/
I can also direct you to a forum where many had/have Medicare on temporary residence visas.