In this article we study the magnetic properties of the BaNiF 4 compound using ab-initio calculations and classical Monte-Carlo methods. We discuss the physical origin of the long range magnetic order. We show that the leading exchange interactions are those of a non-frustrated anisotropic two-dimensional Heisenberg system and that long range order and the experimentally observed preferential orientation of the magnetic moments along the b axis appear when local single-ion anisotropy contributions are added at first order. We also show that a canting of the magnetic moments appears at second order and can be written as an effective Dzyaloshinskii-Moriya interaction.

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