I travel around Australia, I find some camper van hire service in Australia,i just want to know is there some camper park from Sydney to cairns for free?
I have travelled along Australia's east coast between Sydney and Byron Bay on various trips in a camper van. Sydney beaches tend to be more crowded and often require payment for parking. Camping is specifically not allowed here. There are council rangers. Do not stay for free until you are north of the Hawkesbury River.
Once you are out of Sydney, you have three basic options:-
Find a car park near the beach. Most of the beaches which are near some town have toilets and an open-air shower near the beach. Park there during the day and you can go for a swim and use those facilities. You can obtain drinkable water from a tap. If you don't leave any rubbish or stay for days on end you are unlikely to be disturbed. I often see rental campers parked near Bar Beach, although I generally tried to stay somewhere less populated.
In less populated areas, stay in a national park campsite. Some of these allow free camping. They have toilets and may have drinking water.
If all else fails, stay in a rest area on the side of the road like this one or this one. These are not great as there tends to be road noise from the nearby highway. They are not scenic places to stay. There can be tank water that requires boiling before drinking. You may not technically be allowed to camp at these, so you should not be setting up your awning and putting out chairs. You can sleep and claim you were too fatigued to continue driving in the unlikely event that anyone asks.
To a first approximation, no, you will not really find any free camper parks in eastern Australia. (I'm told Western Australia has more.) Campervans need space to park, water, electricity, sewage etc, all of which costs money, so somebody has to pay for your "free" camping.
That said, there are some free campsites, they're just few and far between. This site lists a few, although it's nearly unusable since the maps are broken, while Visit NSW has a more thorough listing of paid options.
Of course, you can always just park in some random parking lot, but this is not a realistic option for more than a night or two, and dumping waste anywhere outside an approved dump station will get you heavily fined.