I think this might be a Welcome Swallow. Allow me to consult my bird book (it was a Christmas present)
...yes 15 cm, tail deeply forked, upperparts glossy blue-black...
"Courting males descend on stiffly downheld, quivering wings... Voice: single 'chep' usually in flight; spirited twittering song; rising 'seep,seep,seep'; in alarm, high pitched puny 'seeet'."
My favourite thing about my bird book is the was it tries to convey the voice of the bird in human terms. To be found in most places in Australia with the exception of deserts and the Kimberley Region. This one was flying around the river in St. Lucia and sat still on the front of the CityCat while I raced to attach my zoom lens.
It doesn't say in my book how it got it's name.