There's not a lot of context here so I (nor anyone else) can give a complete response. A lot of the impact from this assessment will depend firstly on the weighting of the task, and the distribution of marks (10-15% below the average isn't necessarily bad if a lot of marks are clumped up towards the bottom, especially since internal marks don't matter, rankings do).
Yes, you can still get a band 6, but there's not a lot of use obsessing about this comparatively poor assessment, focus on your next one instead. I was always told to 'make your next mark your best one yet'; which is one of the sagest pieces of advice I've ever received from a teacher. If you can boost your rank with subsequent assessments your ATAR will not take much of a hit (especially since Trial HSC has a huge impact on your rank). But as I say, it's not a huge deal since a) ATAR is not the be all, end all and b) thinking about things you can't change is only detrimental to future performance. The fact that you say you were previously in band 6 range proves you CAN do it and have done it previously, what's to say you can't repeat that again? Back yourself.
If you ever need any extra help, pop over to the Biology Question Thread, or any other relevant thread
Good luck!