The big rush starts June 1st traditionally, or after the school holidays.
Those places closer to Perth fill up fast, and then it stretches out up the coast to Derby. Broome in particular gets hit very hard when the population swells from 15,000 approx, to 50,000 approx.
The race season is the busiest time, and the crowds don't move out until September when the weather gets hot as the build-up builds up and the humidity climbs with the temperature.
The parks in Broome don't normally take bookings. It's first in, first served, and the 4 or 5 overflow areas also fill very quickly from mid to late June.
The annual parkers run to schedule from June1, and the other straggling travellers fill the gaps.
Some of the annuals have had the same site in the same park for more than 20 years, so when they no longer make the trip someone else is lucky enough to get a site.
Bear in mind you'll be paying more than $40 per night in the parks. Less in the overflows where you can park for about 3 days. If you still want to stay on you then go to the park which has a spot for you. They're not so strict on the overflow time limit these days because the town simply can't cope with the masses.
Get an early start if possible. Firstly because it's the best time to see the country immediately after the wet, secondly, you'll get a head start on the rush.
Keep an eye on the weather and the school holiday schedule and head north as soon as you can get going. If the wet has slowed down and the roads are open, mid April will be your prime time this year.
Safe and happy travels.

Monkey Mia was very disappointing for me. The hotel and van park are hard to get intoand expensive. I stayed at Nanga Bay "Resort" 80 kms away and went for a drive. I then paid $7 to park my car. This permit is valid for 24 hrs, so I went back the next day, but the dolphins didn't appear for feeding.
Apparently they are reliable for the early morning feed, but not so reliable for the other 2 feeding times.
If you want to see dolphins up close and personal Bunbury does a dolphin cruise.
I didn't get to Kalbarri because it was school holidays and yI had no hope of getting in then.
I think Kalbarri would be the preferred option.
If you're keen to watch dolphins, if ever you get to Adelaide go to Port Adelaide and watch the dolphins on the Port River or take the cruise to see more. Dolphin cruise past the wharf at the boat haven in North Arm for hours. Just love it!

I was in the region in 2007.
The Nanga Bay Resort has everything from camping to caravans, dongas, cabins and motel units. Drinks around the fire pit in the cold weather. I was there in August and the evenings were getting cold. Only a few metres from the beach.
Oh, did I mention the thermal pool? Oh sooooo nice to soak in an artesian pool.
It's an interesting region, but I'd give Monkey Mia a look but not a stay.
Denham is a quiet little village with fuel and tucker supplies if required.

We must disagree about Monkey Mia. We stayed there in 2009 for approx 2 months and loved every moment. We have a tinnie and the fishing was great and our fellow campers were lovely. We will be there for about 5 weeks March/April this year and are really looking forward to it.

After MM we go on to Carnarvon, Exmouth, Coral Bay and then onwwards and upwards UNPLANNED. We will eventually end up in Mission Beach at end July.

Denham has a lovely caravan park up on a hill - great views over the bay. If you have a 4WD vehicle, make sure you explore the Peron NP.
Also recommend a stay at Barn Hill. It is south of Broome - at night you can just see Broome's lights across the bay.
Our experience is that the crowds right up north really build from early June - until late August, in most places. Broome does not really empty much until the end of the Shinju Matsu festival - timing of that is moon dependent - late Aug or into Sept.
The crowds do not stop at Broome - especially if you are looking for powered sites in caravan parks. I have known the main CP in Fitzroy Crossing to be full (powered) by early afternoon, in July. It can similarly be hard to get into Kununurra, though we have always been able to book ahead there (a week or two before arriving).
If you are going on to the NT, Darwin can be hard to get into over July, especially.

MikeMin wrote:

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Hi Jon and Hylda,
Keep an eye out for us at Monkey Mia - we are there on 5/3 - would be nice to catch up with a fellow forumite.
Mike and Minnette

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Hi Mike & Minnette!

We should be up there about the same time as you so with luck we may meet up. What place do you recommend to stay at?

I took a peek at your website though haven't read much of it yet, I did like the pics. Noticed you're originally from South Africa, Hylda also comes from S Africa though hasnt lived there since 1988.

Cheers
Jon & Hylda