Sometimes known as the Madagascar periwinkle (or vinca), Catharanthus roseus has simple, flat flowers usually white, purple or pink, and enjoys Sydney's hot, humid summers, not wilting even on the most challenging days. It gets to about 60 cm tall. I grow it in full sun, and it is tolerant of dryness. It is said to be able to be grown in part-shade in warm climates. There are other flower colours: I have seen a lovely deep purple one in the garden of a friend (pictured above), and some are bicoloured with a distinct 'eye'.
Catharanthus roseus belongs to the Apocynaceae family of plants, and is actually classed as a shrubby perennial, though it tends to become straggly over winter in Sydney and is probably best started anew each year from seed in late winter or seedlings in spring. It may self-seed from year to year; these can be transplanted to other places in the garden if required. I have found that tip-pruning the young seedlings resulted in better-shaped plants. Like many plants in the Apocynaceae family, Catharanthus contains poisonous alkaloids, but it is showing promise as a source of a drug for treating cancer. It comes from Madagascar.