The precise knowledge of the neutrino flux composition and kinematics is one of the biggest challenges of long-baseline experiments such as T2K. Neutrinos are made by the in-flight decay of unstable hadrons produced by the interactions of 31GeV/c protons in a long graphite target. Mostly π (+) (π (−)) are created, leading to the ${\nu }_{\mu }({\bar{\nu }}_{\mu })$ enhanced flux. As kaons and muons are also produced, an irreducible background of electron (anti-)neutrino is also present. The main source of uncertainty in the flux prediction is driven by the lack of data on the proton-carbon interaction in this energy range. The measurements performed by the NA61/SHINE large-acceptance experiment at CERN are used by the T2K collaboration to improve the flux predictions. Two datasets have been taken: using a thin target to study the primary interaction, and a replica of the T2K target to account for the re-interactions. The recently released differential multiplicity distributions of π (±) along the replica target measured in NA61/SHINE will be presented. This dataset is now in the process of being used by T2K to further tune the flux prediction as 90% of the neutrinos will be directly constrained.