My initial thought regarding this was to change schools. I am assuming that all the normal avenues have been exhausted - i.e. reporting incidents to the school, etc. However if they haven't I would do the following if I was in this predicament with my son/daughter. Firstly, you (or your daughter) need to document every incident of bullying that occurs - date, time, who, what, where, etc - call it the "Bully Book" if you like. You need to put your concerns, with reference to specific incidents, in writing to the Principal of the school (if you haven't already done so). You need to follow up with the school and find out what their action plan is going to be. Every single school has an anti-bullying policy and a zero tolerance towards it. Quote info from the Dept. of Education -
https://www.det.nsw.edu.au/policies/student_serv/discipline/bullying/guidelines.pdf
https://www.det.nsw.edu.au/policies/student_serv/discipline/bullying/PD20100415.shtml
Request to speak with the school counsellor/student welfare officer and make an appointment for yourself and your daughter to meet with them. Ensure that the situation is addressed and follow-up frequently and that your daughter receives ongoing counselling at school. Report to the school weekly if need be with facts from the "Bully Book". It is the school's duty of care to keep your daughter safe from bullying and they would be in breech of this code if they do not address the issue.
I would also take your daughter to your GP and document the bullying and the effects that this is having on her. See if he can get a mental health plan and refer your daughter to see a psychologist for therapy. If she is already suffering with low self-esteem she will need support to build up resillience in her. Resillience is important. You/her may not be able to prevent all forms of bullying but you can build your daughter up to withstand it as best she can. A psychologist would be able to give her useful strategies as well as address her low self-esteem issues.
Keep hounding and pestering the school and if they continue to ignore the issue or appear to not be addressing it appropriately I would move up the ladder and take the matter to a regional level within the Dept. of Education.
In the meantime ensure that your daughter remains as active as possible and try to provide diversional activities to free her mind from the issues at school.
This is a serious matter which can have devastating effects on an individual. You have every right to persue the matter further and demand a resolution to it.
All the best
Janna