How do you stop the Australian wildfires?
As of 7 January 2020, the total area burnt by the fires in the whole of Australia is 8.4 million hectares (21 million acres; 84,000 square kilometres; 32,000 square miles. That is equivalent in area to the nations of Austria or the United Arab Emirates. Greater in area than the Czech Republic, Ireland or Sri Lanka.
The vast majority of that has been on the eastern side of the country, in New South Wales (49,000 sq km) and Victoria (12,000 sq km). In Western Australia, the area has been 15,000 sq km and has forced the closure of the southern road between east and west twice.
With many fires burning in many areas the only course of action is to let the fires burn out by themselves. The efforts of fire fighting crews is to mainly protect areas from being burnt: properties and ecological regions of significance, such as the region containing Wollemi pines and other forested areas.
Dumping sand, as you inquire about, has problems: from where to source the sand, the amount of sand required, how to dump the sand, will the dumped sand have a negative environmental impact afterwards.
The fire fighting methods currently use in Australia are the same as with fire fighting practices for bush fires (wild fires) elsewhere. Fire are left to burn themselves out. The fringes of fires will be doused with water, either by ground crews and water trucks or via aerial bombardment, with water bombers (fixed winged aircraft or helicopters). Beating fire out is also used. Aerial bombardment will also include the used of fire retarding chemicals.
To prevent fires from spreading, firebreaks are also made, where large areas of ground are cleared of flammable material to prevent them from being ignited by ember attacks. This can also include the use of back burning, by fire crews, where forest is deliberately set alight by fire crews. This is only done when considered necessary and when weather conditions are favorable - wind directions and strength, humidity and temperatures.
The trouble with bush fires in Australia is eucalyptus is a significant species of Australian forests.  Eucalyptus requires fire to regenerate. They also shed large quantities of leaves and bark that accumulate on forest floors. A controversial method of minimizing bush fires has been to burn forest deliberately in cooler periods, under more controlled conditions, to reduce the fuel load during the main summer fire season.
By not letting the fires burn themselves out, the unburnt leaf and bark litter on the forest floors will remain and accumulate presenting another fire problem for the future.
Australia will always burn, it's the nature of its forests. Bush fires are an expected, but unwelcome part of summer in Australia. What Australia needs is better forest fire management strategies.

Edit 28 July 2020
Recent information about the Gosper's Mountain mega blaze that burnt more than one million hectares.
Anatomy of a 'mega blaze' - text and short videos of about the fire, from ignition by lightening strike, advance, merger with three other fires and how it was fought.
Gosper's Hill 'mega blaze', a 13 minute long video.