As a maths text book, in an absolute sense this is quite good; I like it well enough. As a textbook for standard level maths, however, it is hopeless.
Before we begin, let's try and get an idea of your typical standard level maths student - we'll try to capture something of their essence, if you will. In Australia, given that they chose to do IB, it is likely they will be of above-average intelligence and also be more interested in academic pursuits than a typical non-IB student.
Given, however, that they chose to do standard level (henceforth abbreviated as SL), the vast majority of them will be doing maths for one of two reasons: because it's a prerequisite for a ton of stuff at university or because it's a requirement of the IB. Some of them may enjoy maths in a mild sort of way, but it is highly unlikely that any of them will really love it. The SL maths student is a student who is unlikely to be willing to go to great lengths to understand maths. This is a student who is here simply to get a 7, and if that is not possible, to get the next best possible mark.
This textbook is completely and totally unmatched to this type of student. It uses language that is far above and beyond what is used in class and in exams. The theory sections continually make leaps of logic that are far too large. They skip too many steps, which means you (or I, at least), really have to think hard to understand things that are really not that difficult. There are not nearly enough easier exercises that simply drill drill drill; there are far too many difficult questions which tend to discourage rather than aid. The misleadingly-named 'Paper 1- and 2-style questions' are not at all in the style of papers 1 and 2; they are at least an entire degree of magnitude more difficult.
Your average SL maths student is going to be discouraged from maths for the rest of their lives by a textbook like this. The average SL maths student needs to be coaxed into liking maths; they need to be warmly ushered to the good bits and sheltered just a little from the bad bits. These are students who have the potential to be good at maths; they will never be geniuses (genii?) but they could be engineers, scientists, statisticians. Through maths, they could be capable of making a contribution to something, of making something more. This textbook may have been single-handedly responsible for the destruction of many careers before they even had a chance to take off.
To give an indication of the standard of this book, I did SL maths and found it easy. But I find this textbook hard to very hard to understand. Assuming that a student's skill at maths is roughly proportional to a student's ability to understand maths textbooks (a good enough approximation for our purposes, I believe), if we assume that the average SL maths student would find the subject moderately challenging, that means that the average SL maths student is going to find this textbook extremely difficult to almost impossible to understand.
However, it would be unfair to blame everything on the textbook (this is where I start to rant about things unrelated to books): our education system SUCKS BALLS IN A MAJOR WAY. I don't think it's unreasonable to expect maths students of ages 16-18 to follow the theory sections of books such as these, but the truth is that we can't because they don't teach us to do it. Right through primary school and secondary school right up to probably second or third year university (I'm guessing) we are spoon-fed maths, and to a slightly lesser extent the physical and chemical sciences. The teacher stands at the whiteboard and writes on it; we copy down notes in our exercise books like happy little drones. There is not nearly enough emphasis on teaching us to think for ourselves, to reason things out systematically and logically; instead, we are shown formulas and told to plug numbers into the gaps. Today I worked out how the chain rule works. It was good for me. I felt like my brain was breaking into new territory, forging new neural pathways that will make working out what makes the next formula tick an easier task. But I'm nineteen! My brain is practically calcified by now. Imagine how much more I - we - would be able to do if we began doing this sort of thing at age five, when we started school. How much more innovative the world could be!
But I know it's much easier to tear something down than to suggest a workable alternative, and goodness knows I don't know how else it can be done with any degree of practicality and efficiency.