There's a few reason why the ATAR score is quite low for science. First of all, Monash offer two advanced science courses, one to those with atar over 99, and those with atar over 95. Hence the high achieving students use those courses and thus don't bring the regular science degree atar up.
Also Monash offers a lot of double degrees with science that usually have 90+ atar scores.
Once you eliminate those high achieving science students and the double degree students, there ends up being more places available for those wanting to do straight out science hence the atar ends up being low.
In terms of actual subjects available, Monash appears to be really good at teaching Chemistry and Biology (which I assume you'll do if you're interested in Medicine.) Their physics department is terrible but their Maths department is quite good.
If you do straight science, you will have to do 1 unit of mathematics, however this can be anything such as the Methods equivalent or a statistics subject that helps you with other areas. There is also a compulsory science subject which most students tend to not like.