What prefix do you need to call a US mobile using a Australian mobile when in Australia?

00 61 from UK, and drop the leading 0 from the Australian number.

On a GSM mobile phone, dial +61 (including the plus symbol), followed by the Australian number without the leading zero.

Incidentally, you dial the Australian mobile phone this way, wherever on earth the mobile is physically located (Australia, UK, or anywhere else).

(The plus sign means "insert your international access prefix here." From a GSM mobile phone, you can enter the number in full international format, starting with the plus sign. The most common prefix is 00, but North America (USA, Canada, etc.) uses 011, Japan uses 010, Australia uses 0011, and many other countries use different prefixes.)