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The ultimate goal in volcanology is to precisely forecast volcanic eruptions. One step towards this objective is to discriminate between processes of magmatic and non-magmatic origin. However, geophysical and geochemical anomalies are ambiguous and can be attributed to a variety of subsurface processes. This thesis aims to advance the understanding of unrest signals and underpinning processes in different volcanic settings, specifically two crater lake volcanoes in New Zealand. For this, I use Multiphysics modelling and forward model ground displacement, gravity, and self-potential anomalies to identify measurable signals from magmatic and hydrothermal unrest. First, I study magmatic unrest signals and test the sensitivity of self-potential to strain-induced fluid flow in volcanic aquifers by using a novel modelling approach. I was able to determine distinct patterns of self-potential anomalies and surface displacements from magmatic stressing of dike and sill intrusions. Then, I investigate magmatic and non-magmatic unrest signals at the crater lake volcano, Mt. Ruapehu in New Zealand. Model results show unique sets of measurable geophysical signals from either magmatic or hydrothermal perturbation, providing valuable information for monitoring efforts of the high-risk volcano. Lastly, I compare surface displacements from satellite observations with hydrothermal unrest models at Whakaari/White Island; with some models matching observed displacement magnitudes. Furthermore, I find detectable self-potential and gravity changes from hydrothermal fluid fluxes which advances the understanding of potential geophysical signals at Whakaari/White Island. The results demonstrate that magmatic and non-magmatic processes induce markedly different spatio-temporal unrest signals. I demonstrate that self-potential and gravity changes are useful indicators for interrogating source mechanisms behind volcanic unrest, especially when combined with common monitorin