Hey there,
I am in the science extension course as well. My mentor is my teacher, who has been instrumental in guiding me through my research project. I also have another (sort of) mentor from Macquarie University as well. I would say that it is not detrimental if you don't have a mentor, but just super USEFUL.
In saying that, there are plenty of resources online about how to tackle a science project from start to finish. You shouldn't rely on your mentors for everything. I think I have relied too much on my teacher, so I don't really know the ins and outs of my project. I am trying to change that with more research, but it should not be a problem for you.
It is best, however, to have someone who knows about your project and the respective field of science in depth.You should try (with permission from teachers) to contact people from universities. Just send them an email detailing what your project is and your doubts etc. My teacher and I contacted one person at Macquarie University for CO2 monitors, and he referred us to someone else, who came by to see my experimental setup. He gave some really useful tips for improvement and gave us advice on how to take it to the next step. I don't think it is too late, so if you already have a developed methodology for your project, you should try to find someone who can critique and eliminate potential errors from it. You don't have to do this, but it is nice.
Your teacher should also be looking over your project and give you some solid advice as well.
Hope this post helps!!!