In the framework of a continuum model based on the microscopic magnetic symmetry properties of the metal oxide compound La₂NiO_{4+ delta}, a theoretical analysis is made of the stability of the antiferromagnetic (AFM) long-range order in the presence of impurity centres of excess oxygens at delta <<1. The resulting magnetic phase diagram in the variables delta and T is compared with the recent experimental data for this system. Also the similarities and dissimilarities between the structurally isomorphic systems La₂NiO_{4+ delta} and La₂CuO_{4+ delta} are traced and their possible connection with superconducting properties is mentioned.