Marine heatwaves are prolonged, extreme warming events that have been shown to cause severe and lasting ecosystem damage and have recently had severe economic consequences for the seafood sector. To date their investigation has largely been constrained to surface studies, and primarily in deep ocean waters offshore. However, it has recently been shown that the sub-surface structure of marine heatwaves can be complex, and not necessarily well represented by surface fields. Here we characterise the sub-surface structure and variability of extreme ocean temperatures in key fishing grounds around New Zealand. We explore the decoupling between offshore and coastal warming and we demonstrate that understanding this complex and varying structure is important for predicting ecological impacts of MHWs. This is of particular significance for the lucrative commercial seafood sector who focus much of their effort on (and derive profit from) shelf and slope waters to ~ 500m