I happened to catch this take-off from the "wrong side" - the shadow side - of this 767 departing Sydney runway 34R bound for Melbourne. Notice the stream of vapour from the rear. It appears ATC saw it also and advised the crew that it was "something." The aircraft returned to Sydney as a precaution. See the next photo for a better idea of what ATC would have seen.

As it turned out, it was, indeed, water coming out of a burst line or leaking shut-off valve.