While serving, Coster studied law at the University of Auckland,[6] and briefly left the Police to work for Meredith Connell as a Crown prosecutor.

[7] After attaining the Wellington-based position of assistant commissioner for strategy and transformation in 2015, Coster was seconded to the Ministry of Justice in 2016 where he was a deputy chief executive leading a court reform project.

[7] Returning to the Police in 2018, Coster was an acting deputy commissioner, during which time he was involved with weapons reforms following the March 2019 Christchurch mosque shootings.

[18] That September, it was announced Coster would leave the role sooner to start his next position as chief executive of the government's new Social Investment Agency in November 2024.

[20] In contrast, President of the Police Association of New Zealand, Chris Cahill, took a different view commenting that Coster's resignation was "probably a good call" and that he "hasn't been everyones favourite commissioner".