Using parts of previous work in dissertation I'm currently getting towards the end of my Master's year, and so I am working on my dissertation.

I completed a couple of projects for a module (assessed by report) in my third year that are relevant to my Dissertation, which include the geological/geophysical background to the region, and also the background theory of the methodology. These, however, are not published pieces; they are written coursework reports.

So these sections for my Dissertation are essentially written in different reports.
My question is whether it's acceptable to assimilate this information in my Master's write-up? (Mainly the methodology, there aren't many ways to concisely convey the information and theory, c1000 words.)

This probably matters, and my dissertation supervisor is the same person who marked the other pieces of work.

Thanks for your help.
The only correct answer here is: Ask your supervisor!

You are not exempt from rules about self-plagiarism just because your previous work was unpublished, but without knowing the material, there is no way for us to be sure what side of the line you are on. Since your supervisor is also the person who previously graded this, it is natural to ask. Your supervisors job is, after all, to supervise.