I'm sure this is an Achelia but which one?

Just as I was leaving work on Monday I noticed this butterfly sitting on a plant in a but of scrubby grass we have along the side of the office. I'm fairly sure it's an Achelia but I don't know which one. The flowers start off as white but then turn pink. I'm not sure where it has come from as it isn't one that had been planted so presume seeds on the wind. If it attracts butterflies I might dig it up and take it home to accompany my Achelia Moonshine.