Grade requirements will differ somewhat from university to university, and you should also note that some schools will note accept the ATAR or OP, and instead require international A Levels or the IB.
Here are some examples.
King’s College London accepts the ATAR for direct entry into its undergraduate programs, including Medicine. You will need evidence of high grades and a pass in the Year 12 High School Certificate of Education. For Medicine, A*AA in the UK would be the equivalent of 97 in the ATAR (with a pass in the Year 12 HIgh School Certificate of Education).
Manchester also accepts the ATAR, and specifies that you must have received a score of at least 98, including Biology or Chemistry and a second science from Chemistry, Biology, Physics, Maths or Psychology in your High School Certificate at a minimum score of 90% or equivalent. They additionally accept the Queensland Overall Position Rank or OP, which must be at least band 1 and, as with the ATAR, include Biology or Chemistry and a second science from Chemistry, Biology, Physics, Mathematics or Psychology in your High School Certificate at a minimum 90% or equivalent.
You must offer both English Language and Mathematics to at least High Achievement or grade B in Year 10 certificates.
UCL does not accept any Australian qualifications, and will only consider those who offer acceptable international qualifications.