You need to look at a document called INF 16 This document deals with immigration rules and how they affect au pairs coming to the UK. It contains a list of countries for which the au pair scheme currently applies.
Australia and New Zealand would fall under the Working Holidaymaker scheme INF 15 .
Canadians can come to the UK as a working holidaymaker, but Americans can not come to UK as either an au pair or working holidaymaker.
For further information about immigration rules and visa requirements, visit ukvisas.gov.uk