Taking a masters in one discipline and changing to another for a PhD

I am currently a high school student, and I am considering taking a BSc in Aerospace Engineering (in Delft), hopefully leading into a masters.

I am also very interested in pure mathematics (I am aiming for the Top Scholar award in Mathematics within New Zealand), and want to do a PhD in maths.

Is there any way to continue my studies in Mathematics beyond high school as well as taking aerospace engineering, and while keeping options open for a PhD in maths?