Doesn't delaying them till third year mean you'd have to take them as Level 3 subjects? Is that even available as I thought they were only Level 2 subjects? Is my understanding of levels wrong?
You can definitely do level 2 subjects in third year. Level 1, 2, and 3 does not necessarily mean they need to be done in Years 1, 2, 3 respectively. As far as I've understood, for every degree, there's a certain number of credits (i.e. subjects) that do be completed at each level. The requirements for bachelor of Science, for example, can be found here:
https://handbook.unimelb.edu.au/view/2017/B-SCI under 'Course Structure & Available Subjects'.
So the levels refer to that.
And do most people do the "Techniques in Molecular science" subject concurrently with Biochemistry?
Thanks 
They can be done together, or separately. From what I know, most people do tend to do them concurrently, but I did Techniques in Molecular Science in 3rd year as an elective/selective, and completed Biochem the year before. I didn't at all find it a disadvantage, so really up to you.