You can do optional modules in every year. You have to do at least 120 CATS, but you can do up to 150 if you want.
First year has 100 CATS of core modules, so you can pick between 20 and 50 CATS of optional modules (there are optional maths modules, but these can also be from pretty much any department). For context, that's 9 core modules, and then each of the optional modules are generally 10-15 CATS each.
Second year has slightly more complicated regulations, but you have 55 core CATS (5 core modules), and then another 20 CATS of optionals from more restricted lists (2 semi-optional modules), but then the rest are completely up to you. Again, the extra optionals are generally 10-15 CATS, so you can fit in a lot of choice.