I feel like being pedantic for a moment. Our suburban posties don't get off the bike to deliver the letters, they ride up alongside the letterbox and deliver the letters right from the vehicle, as can be seen in the link in the GGP comment. They also deliver small parcels that will fit in a standard letterbox.

You are quite right though, as soon as the rain starts, they're in full wet gear. Bright yellow, you can't miss them.

Suburban parcel delivery is specifically taken care of by Startrack, an Auspost subsidiary, usually in a trusty Toyota Hiace van.