<p>In this work, we reconstruct the geometry and magma properties of the dyke that fed the Nisida eruption (~3.9 ka), located across the rim of the Campi Flegrei caldera (Italy). Results indicate that the magmatic overpressure was ~35 MPa, the dyke depth of origin was at ~5-6 km, consistent with petrologic studies, and the volume of the active magma chamber was 85-150 km³ with an upper bound of excess pressure rupture threshold of ~9 MPa. We show that a range between 4-9 MPa is common among the moderate-sized volcanic eruptions vented along caldera structures, suggesting a shared magma reservoir. Considering the volume change required to reach the rupture threshold, we simulate the ground deformation pattern triggered by such magma recharge, which produces a different and thus distinguishable geodetic signal compared to the currently observed one. This work may serve as a reference for identifying the transition from magma-driven unrest to volcanism at Campi Flegrei.</p>