caravan park acommodation

Yes

Very good idea too. Prime travel season for all heading north and west. If you're a member of the various park groups (Top Tourist, Big 4, Family parks etc), they can usually book ahead for you.Places like Broome, Cape Leveque, Kununurra & El Questro you would be best to enquire and book well in advance. Even Darwin can be hard to get a spot in at that time of year.

Cheers Mick

As we travel nomadic style, we never book. We got to Kununurra peak season and show week and it was tight for powered sites (show grounds is usually used for overflow but was not available that week), but park we went to had plenty of room in the non powered area. We do not go to 'resort' type places such as Broome (although we got a site when we went five years ago), so it depends where you plan to stop, whether you need power, whether you can 'bush camp', and whether you can cope with a regimented schedule. It sounds like you will have a extremely tight schedule. We have not been to Darwin.

Motherhen

who spent 9 weeks in the Kimberley and 5 weeks around Alice Springs last winter

In 2007 we were able to get into a good van park in Alice Springs in mid June, by phoning 2 days ahead. School holidays across late June to end July will cause your greatest problems - everyone is up north then!! So Darwin may be an issue for you. You may do best by trying the more outlying parks like Howard Springs rather than those closest to the city. We got into a Kununurra caravan park in late June by, again phoning a couple of days ahead - we were lucky though, because it rained heavily in the area at that time and a couple of hours after we arrived to commence our stay, there was not a site to be had in the town for love nor money, and it stayed that way for a couple of weeks. Broome will cause you big problems - even the overflow areas, which are not great, get full through July and August. We were very lucky, phoned about 3 weeks ahead and got a caravan park cancellation that had just been made. This was August, and they would not take a booking for anything less than a week (which was longer than we wanted).
I guess what you do will depend on how much certainty you require about accommodation, in advance, or whether you are ok with "winging it" at the time.

I've never booked ahead for van parks, but usually don't need powered sites.

Bullbar,
I hope your not spending your kids inheratance ????
I think you should just go to Adelaide for the weekend.