Constrain axis of isometric camera ("vertical" only)

I would like to make a camera scroll through an isometric scene up and down but not left to right.

I was wondering what the best approach to this is. I've tried reseting an axis to 0 on update, but as the camera is looking down onto the scene at an angle already, it's not constraining correctly.

I was thinking either make the camera a child of a null object and move that, as it's axis are not rotated already. Or find some interesting way to constrain the x and y at the same time that allow for vertical scrolling only. One idea is to minus the values from the axis, not tried this yet.

Is there an existing method?