I don't know why the
Perth Mint didn't make pre-decimal silver.
BUT! I think I have a spectacular guess:
1. Sydney Mint is producing silver coins during the "roaring twenties" - helps Melbourne Mint cope with the load.
2. Sydney Mint shuts in 1927-->
3. No silver coins produced 1928-1930
4. Depression hits in the 1930s and mintages for all coins fall, Melbourne Mint can cope.
5. US Mints produce coins during the war. I can't explain why the P.M doesn't.
OK, perhaps guess wasn't spectacular, but there ya go, maybe a few factors why -