I see.
I do not have the figures to back me up, but I would think there are far more A-Level candidates than there are for Australia's Year 12 candidates, which means top 5% in A-Level is not the same as the top 5% of Australia's Year 12. The top 5% in A-Level could very well be far better than the top 5% of Australia's Year 12.
Consider this. There are 10,000 A-Level candidates and 5,000 Australia Year 12 candidates. (example figures)
Top 5% of A-Level candidates = Top 500 candidates.
Top 5% of Australia Year 12 candidates = Top 250 candidates.
I believe it is significantly harder to best 9500 candidates (A-Level) than to best 4750 candidates (Oz Year 12).
However, it sounds ridiculous to say that the top 52.5% of A-Level candidates is equal to the top 5% of Oz's Year 12 candidates (better than 4750 candidates).
Therefore, I believe the percentage of candidates that score certain grades cannot be given an ATAR score based on the percentage only.
So have you been sent notice for interviews yet? It should be around this time.