An overall survey of the structural and magnetic features of the La₂NiO_{4+ delta} system is presented as a result of neutron diffraction experiments. The stoichiometric compound ( delta =0) presents two structural phase transitions. At T₀ approximately=770 K, La₂NiO₄ transforms from tetragonal (I4/mmm) to orthorhombic (Bmab); at T₁ approximately=80 K, from orthorhombic to a new tetragonal (P4₂/ncm) phase. Associated with this second phase transition a strong microstrain produces anisotropic broadening of Bragg reflections. La₂NiO₄ is three-dimensional (3D) antiferromagnetically ordered at room temperature (T_N=330 K). A weak ferromagnetic component appears below T₁. Oxygen excess suppress the 3D magnetic ordering and the structural phase transformations, giving rise to a non-stoichiometric compound with interstitial oxygens. A tentative phase diagram is proposed.