We present the results of first principles molecular-dynamics simulations of nitrogen vacancy complexes in monolayer hexagonal boron nitride. The threshold for local structure reconstruction is found to be sensitive to the presence of a substitutional carbon impurity. We show that an activated nitrogen dynamics triggers the annihilation of defects in the layer through formation of Stone-Wales-type structures. The lowest energy state of nitrogen vacancy complexes is negatively-charged and spin-polarised. Using the divacancy complex, we show that their formation induces spontaneous magnetic moments, which is tuneable by electron or hole injection. The Fermi level s-resonant defect state is identified as a unique signature of the ground state of the divacancy complex. Due to their ability to enhance the structural cohesion, only the di-vacancy and the nitrogen vacancy carbon-antisite complexes are able to suppress the Fermi level resonant defect state to open a gap between the conduction and valence bands.