What Are These And Value? | Romano-Egyptian Tetradrachms

Man, are they ever a long way from home? It's probable that one the moderators might move this query to the Ancients board where there's a bit more expertise.

As for "what they are", they are base-silver tetradrachms from Roman-occupied Egypt; the one on the right is emperor Claudius II and dated Year 2 of his reign, which would convert to AD 269. Click on this Wildwinds page and scroll right down to the coin labelled "Alexandria Milne 4252".

The one on the left is harder to read and I can't ID it from those pics, beyond saying it's a similar coin from the same place and similar time period (roughly AD 250-300).

You definitely shouldn't be finding them metal-detecting a backyard in Australia. They certainly didn't arrive there "naturally", dropped by some passing Romano-Egyptian trader 1800 years ago. Somebody's lost or otherwise disposed of an ancient coin collection in your uncle's yard, and fairly recently - these debased-metal tetradrachms are made of an alloy that corrodes very easily and would start to degrade severely after just a few years of exposure to a typical suburban yard environment.