The nuclear and magnetic structures and the magnetic phase transition of Ni311B7O13Cl[Ni-Cl] boracite were investigated by high resoln. and high flux neutron powder diffraction at 1.5-30 K. The temp. dependence of the magnetic intensities shows an unsharp onset at TN = 9 K, indicating the existence of a 2nd-order magnetic phase transition. Refinements of the magnetic structure give rise to the magnetic space group Pc'a21' for the magnetic phase below TN, with the magnetic modes Cx, Fy, and Az for the 3 Ni sublattices. The 3 Ni sites display 2 kinds of magnetic moment with different values and orientations: Ni(1) has a moment parallel to the main antiferromagnetic axis along [100]or, while Ni(2) and Ni(3) have their antiparallel moments along [010]or directions. The total corresponds to a coplanar 2D canted antiferromagnetic arrangement, which is characteristic for the magnetic structure of Ni-Cl. The refined magnetic moment values at 1.5 K are: μ1x = 1.65(2) μB for Ni(1), and μ2y = -μ3y = 0.79(4) /μB for Ni(2) and Ni(3), resp., showing a weak magnetic superexchange interaction compared with the spin only moment 2 μB/Ni2+ observable by neutrons. The magnetic ordering process and the possible frustration effects due to the interwoven configurations between the 3 Ni sites and the formation of isolated Ni3Cl groups, are discussed based on the proposed magnetic structure.