I'm an ex-Wellingtonian, now lurking down in Dunedin. Your bakes are looking very similar to results I was getting in my home oven. I tried a lot of things to get some charring on the underside, and nothing really made any difference until I bought a little counter-top oven second-hand on Trademe (one of these: http://www.productreview.com.au/p/breville-pizza-pronto.html
). The stone in the oven gets really hot, I've clocked it at over 900f but don't usually go that extreme.
My process is to start the bake in the little oven, char the bottom a little after 60-90 seconds, and then transfer to the main oven which I leave on grill, to finish the top side of the pizza. That might seem like a bit of messing around, but it's not at all hard to do and is actually quicker from start-to-finish because you don't have to worry so much about pre-heating the main oven, the grill does all the work, not the stone.
I've got a thread over here: http://www.pizzamaking.com/forum/index.php/topic,20213.0.html,
if you follow the photos you'll see a few pizzas that have very similar crust colour to what you're getting, and then the last pics (as at May 4 2013) are using the oven-swap approach.
[EDIT] - darn I've just read that you don't have a grill in your oven!! Ok so maybe this technique doesn't apply