"There are a lot of monarch butterflies here in New Zealand.
They can be found mostly where-ever there are swan plants.
I don't know whether these plants are the same as your
milkweed, but swan plants get a large number of pods, in
the shape of a swan (where they get the name) and are full
of seeds. Swan plants can be brought from most garden
centers. I have a few plants, so there are always monarchs
around in the summer months." - Steve