Fluid flow and pore-pressure cycling are believed to control slow slip events (SSEs), such as those that frequently occur at the northern Hikurangi margin (HM) of New Zealand. To better understand fluid flow in the forearc system, we examined the relationship between elastic properties, compaction, porosity, and permeability of Cretaceous-to-Pliocene sedimentary rocks from the Raukumara peninsula. We found that the permeability of the deep wedge is too low to drain fluids, but fracturing increases permeability by orders of magnitude, making fracturing key for fluid flow. In weeks to months, plastic deformation and clay swelling heal the fractures, restoring the initial permeability. We conclude that overpressures at the northern HM might partly dissipate during SSEs due to enhanced permeability near faults. However, in the weeks to months following an SSE, healing in the prism will lower permeability, forcing pore pressure to rise and a new SSE to occur.