WA

Got no idea where you are starting from, but it matters little. That part of Australia is great, so it is as good as any to go via Katherine.

Probably the best way to go to catch the Wildflowers at their best as you head south through WA. August september is usually the best time for the wildflowers.

Yep. You cant dilly dally too much in the north though. Ideally it would best to be leaving Broome mid June, depending on the weather. Ring ahead to small town tourist centres to get the good guff on wild flowers.
Dont miss Karijini .
enjoy.

Neil

wildflowers all depends on rainfall, was out landor/mt augustus in july doing survey (vollie) work, and not much happening. over at coast flowers going strong early august, all way along the coast with fields of everlastings along hwy edges.

had the ability to go again up landor/mt augustus mid sept and got back 2 weeks ago so away approx 1 month. flowers were plentiful around perenjori on way up, including leschnaultia wreath flower.

By time i was heading south along hwy from carnarvon all roadside everlasting were gone.

if running diesel try fill up at unmanned sites they usually fair bit cheaper than manned one even in towns -at time 2014 kununurra ord river co-op had diesel at 179.9 compared to 199.9 in town.

Few 24 hr overnight stops between towns to save bit of $$ between kununurra and broome -then same goes for broome to Pt hedland - no water supplies in broome for travelers either so may be good idea to be in caravan park to refill water tanks.

if you can do a scenic flight out of kununurra to bungle bungles and incorporate wolf creek crater in process cheaper from there than a local flight out of halls creek -just gives a different perspective to seeing from ground.

We travelled south from Millstream National Park at the end of August this year and found masses of wildflowers all the way to Perth. The biggest concentration was around the Mulewa Region which we visited in mid September.
All the local tourist information centres have detailed knowledge of the best spots to visit and the WAust Parks and Wildlife publishes many informative books/pamphlets, one of the best being "Your Holiday Guide to Western Australia's Wildflowers". (probably available on line)
As others have said the annual rainfall is a major factor as to how many flowers appear. This year was well above average.