Pea flower more canary than butterfly
This is a canary, and these are the flowers of the Canary-bird Bush.
You won't get the two confused but you can see, perhaps, the point of similarity in colour and bearing. With flowers this large and showy, a bright little bird is not a bad analogy. For a plant botherer like me you can't help but wonder anew at the form and function of a pea flower, often described as papillionate, or butterfly-like.
Some flowers of the pea family are more like butterflies than this one but it has the typical bilateral symmetry, where there is only one way you can slice it to create two identical (but mirrored) bits. We call this zygomorphic.
The big petal sticking out the top is commonly called the standard, or sometimes banner, and less commonly these days (unless you read obscure taxonomic journal articles) the vexillum. It's the part that when broad and notched in the middle (in other genera) can look a little like butterfly wings. Below this is the keel, formed by two narrow petals at least partly fused together along their length, flanked by two wing petals. The reproductive goodies are gathered together inside the keel.
In the flowers of this Crotalaria agatiflora, the Canary-bird Bush, from the highlands of tropical east Africa (and the Grey Garden in Melbourne's Royal Botanic Gardens) the standard is spade-shaped, the wings quite small and like little tongues poking out the side of the brown tipped keel.
The Canary-bird Bush is just one of 700 or so species of Crotalaria, most of them native to Africa. Crotalaria is a member of the Papilionoideae subfamily of Fabaceae (or in some systems, simply the Fabaceae with the other subfamilies including cassias and wattles pulled up to family level).
As I said, the flowers are zygomorphic, as are most in the subfamily. There are a only a few with more complicated symmetry, e.g. the asymmetrical Vigna caracalla, the Snail or Corkscrew Vine (its flowers have weird curly bits but you'd probably still recognise these as variations on the papilionoid theme).
Pollination in papillionate flowers is usually 'brush type', where the male and female parts emerge from the keel in response to the insect rummaging inside the flower for nectar. In all my pictures the stamens and styles (male and female bits respectively) are well hidden, waiting perhaps forlornly in Australia for butterflies and bees heavy enough to part the keel.
The colour, the drama, and the size are all about attracting the pollinator and presumably guiding it to the bits that matter. I'm not sure about the dark brown spur on the keel but I bet that guides the insects in some way so that they assist the plant in its pollination.
Finally, the source of these flowers, our Crotalaria bush, a green bush with yellowish green flowers, nestled among grey plants in the Grey Garden. We also grow it in a couple of other places in the Royal Botanic Gardens. Up north, and even in far eastern Victoria, you might find it growing weedy in the bush: it's naturalised widely in the Southern Hemisphere, even in cooler countries such as New Zealand. So beware and be careful if you grow it, but do enjoy its papillionate, or canary-like, floral display.
Images all from the Grey Garden in Melbourne's Royal Botanic Gardens, except for the Canary, which is borrowed from LafeberVet.com.