Ruth, I understand that you are upset re the condition of these horses as any horse lover would, but you must understand that the RSPCA in Australia has very limited power, they cannot just go onto someones property and seize the horses, reports have to be filed, the owner contacted, court documents to be filed in court, court dates etc etc. This process can take months and years to be put in place. Also the RSPCA does not have the resources, finances or man power to care for at risk/high needs horses - they depend on foster homes that take on the horse or donatations (which does not go far). Not many people are able to or would want to take on that burdon financially or emotionally.
If you feel strongly about this situation then contact the local Council, the local MP, the Radio Station, News Reporters, all local Welfare groups, The local Pony Club, etc. This way you will have complete coverage and people will come to your aid and help.
Unfortunately by putting your plea onto this international forum, the good people here will be unable to help and the handful of Aussies that frequent the site are from all over Australia - I myself though would love to help but I am nearly 20 hours away from you.
You obvoiously can use the computer and phone - then start phoning the people I have mentioned and start sending Emails to all political people in that area. Make them aware of the plight and you should get help from the people of your state.
The only person that can help these horses at this time is you.
Also be very careful as you have posted photos of horses that are not your own and you do not have the permission of the owners to do so. Yes the state of these horses are terrible, but you are skating on thin ice legally and this can come back on you.