I'm in an apartment in Hong Kong with a Sony ICF 7600GR tied to a few metres
of wire hanging outside the window as antenna.

Radio Australia is strong as if it is my local MW station. That's probably
because they've relays in Singapore and Taiwan. In contrast, Radio New
Zealand is noticeably weaker although it's still audible. Seems their
signal come all the way from their country with no relay halfway.

I'm very surprised that you could pick them up as far as central US.