Well, looks like I was way off about the election reasons. Rob says this:

This is not a double dissolution. The G-G has the power to dissolve the House of Reps, and does so on the advice of the PM. He can do this whenever he likes, but practical considerations (for example, keeping Senate elections in line with HoR elections) limit the time available to do it.

And Nomes goes into more detail:

This is not a double dissolution election, but a half-Senate. This is the usual state of events - to enshrine state rights only half the Senate goes at once. As each Senator has a two-election term, there is little turnaround, which causes a certain stability. A double dissolution is truly a shake up.

Double dissolutions were a big feature of the Fraser years and the Hawke Government years but seem to have gone the way of the dinosaurs because the Howard Government has been too frightened to use it. You are right that when supply is blocked the Prime Minister can seek a double dissolution from the G-G, and hope to remove the blockage that way, but it's politically risky.

Why can Howard call an election when he wants to? Strictly he doesn't - he asks the GG to dissolve Parliament, and has to persuade the G-G that is is necessary and desirable. Clearly however the GG usually does what the PM wants - except for Whitlam. Three year terms are merely a convention. Fraser went twice in two years.

Thanks for clearing that up, people.