There are usually not that many Band 6 students in Agriculture as it has a relatively small candidature.
Students that do end up with a band 6 have to put in a lot of work.
First, you need to be familiar with the syllabus requirements and check that you have covered all the required content.
Some aspects of the course will need to be memorised, such as the nutrient cycles and energy partition.
Other aspects you will need to become very familiar with, such as the study you need to look at in the elective.
You will also need to be aware of the bigger picture, and how agriculture fits into the Australian economy and the interactions it has with other parts of society - looking at tv shows like Landline and reading ag magazines or newspapers can help with this.
The other thing you will need to work on is exam technique, and it is worthwhile to do as many practice exams as you can find - pester your ag teacher to go through the papers with you and help you work out where you can improve. Also learn the meanings of the key words used in exams so that you are answering the questions in the way that the examiner expects.