It doesn't seem like there's a certain mark needed to be eligible. The History Teachers' Association of NSW mainly requires things that are more relevant to the subject itself, and states the following:
The Extension History Essay Prize is open to essays that:
- Satisfy the requirements of the NSW HSC Extension Project
- Are the product of genuine research
- Represent 'interesting and well-written history'
- Do not present a significant overlap with other senior history syllabus content
- NOTE: Special Category essays will be selected by the judging panel from essays entered in the HTANSW Extension Essay Prize, and must meet the same criteria
The HTA also states the following in terms of your submission:
ESSAY REQUIREMENTS
Essays should satisfy the requirements of the NSW Extension History Course Project consisting of:
- An essay of no more than 2,500 words
- A bibliography (but not evaluation of any of the sources used)
Please also ensure:
- Your essay has page numbers
- Apart from the essay's PDF file name, do not write student's name, school or teacher on any page of the body of the document.
As you can see, there are no set criteria for a specific mark required to be eligible. However, it would most likely be beneficial if your submission received as high of a mark as possible, in terms of having a more competitive submission
I hope this helps!