This one's really intended for students in year 11/12, & their teachers & parents - those of my readers who are in year 13 will already have worked out where they'd like to be & made their subject choices.
In the weeks before the start of the A semester, my diary rapidly fills up with appointments as new and returning students come in for advice about their programs. For the latter group it's usually fairly straightforward: have they met the various pre-reqs? Is their major remaining the same? How many papers do they need to complete their qualification? Things like that.
With new students, helping plan a program can be a bit more difficult, & often this difficulty also hinges on pre-requisites - the subjects (&/or the number of credits in a subject at Level 3 of the NCEA) that you need to have taken in order to study a particular subject at university. For example, for Engineering you need to have a certain number of credits in physics, maths (with calculus) and - for some areas - chemistry. The same's true for Chemistry, Physics, & Maths. While my own subject - Biology - doesn't have any formal pre-reqs, it's still useful to have studied it at year 13 as we do tend to move along fairly quickly & you'll probably feel more comfortable with that if you've got some prior learning in the subject. If someone comes along who's never studied bio at all, I strongly encourage them to take a 'catch-up' course over summer. And while I remember - a bit of maths can never go amiss. Contrary to what many students seem to think, biology is not a maths-free zone & it's useful to have some familiarity with basic maths concepts.
So for someone who doesn't really meet the prior-learning requirements for a particular discipline but is set on studying it, we work with them to identify a pathway that will give them the best chance of succeeding with that goal. That 'catch-up' course is one avenue, after which we follow the tutors' recommendations about what papers the student should be placed in. For someone with insufficient credits in maths or physics for the papers specific to an Physics or Engineering program, we would enrol them in the relevant introductory papers (& on passing those, they'll move to the degree-specific papers), plus encourage them to take up all available opportunities for extra tutorials & other learning support - & in fact I direct students from all disciplines to these resources as & when it seems necessary.
However, all of this imposes extra costs in terms of time & things like student loans, in that it will take a bit longer to complete the desired program of study. And one way to avoid this is to work with your teachers, careers advisers, & the recruitment staff from universities to identify where you want to be in terms of study & careers options and to do it early. Ask around - contact the universities you think you might want to attend to ask about pre-requisites for the degree programs you're interested in, & then work with school to make sure you have the opportunity to achieve them.
(And yes, returning students can & do change their career intentions in ways that have a similar impact, in that they may have to take extra papers - & hence longer than originally intended - in order to achieve their goals. And again, the job of advisers like me is to work out the best options available for them to do that.)