  {\it Ab initio} calculations, GGA/GGA+$U$, are used to propose a spin
Hamiltonian for the B-site ordered double perovskite, Sr$_{2}$NiWO$_{6}$. Our
results show that the exchange interaction constants between the next nearest
neighbors in both intra- and inter- $ab$ plane ($J_2$ and $J_{2c}$) are an
order of magnitude larger than the ones between the nearest neighbors ($J_1$
and $J_{1c}$). Employing the Monte Carlo simulation, we show that the obtained
Hamiltonian properly describes the finite temperature properties of
Sr$_{2}$NiWO$_{6}$. Our {\it ab initio} calculations also reveal a small
magnetic anisotropy and non-trivial bi-quadratic interaction between the
nearest inter-$ab$ plane neighbors, which play essential roles in stabilizing
the type-II anti-ferromagnetic ground state of Sr$_{2}$NiWO$_{6}$.
