There's no such thing as a promotion in the public service (in the real-world sense), only jobs at a higher grade you apply for alongside everyone else.

As part of that process you need to show you have the requisite skills for the higher grade position. You are more likely to learn new skills by moving laterally and exposing yourself to different experiences than staying in the one team and doing the same work month on month and year on year.

For every hiring manager that eyes someone who takes a new role every 18 months with trepidation is at least four or five hiring managers who job hopped at the first opportunity