One of two. The echidna is also a monotreme (although there are 4 species of echidna, while there's only 1 platypus species). All are endemic to the Australian continent (which includes New Guinea).

I've seen both echidnas and platypusses in the wild. Echidnas are cute. But seeing a platypus is quite magical. I can't remember each specific sighting but it's probably only been something like 2 to 4 times. The time I do remember well was quite an extended observation (the other sightings only glimpses).