  Hybrid transition metal oxides continue to attract attention due to their
multiple degrees of freedom ($e.g.$, lattice, charge, spin, and orbital) and
versatile properties. Here we investigate the magnetic and electronic
properties of the newly synthesized double perovskite Ba$_2$NiIrO$_6$, using
crystal field theory, superexchange model analysis, density functional
calculations, and parallel tempering Monte Carlo (PTMC) simulations. Our
results indicate that Ba$_2$NiIrO$_6$ has the Ni$^{2+}$
($t_{2g}^{6}e_{g}^{2}$)-Ir$^{6+}$ ($t_{2g}^{3}$) charge states. The first
nearest-neighboring (1NN) Ni$^{2+}$-Ir$^{6+}$ ions prefer a ferromagnetic (FM)
coupling as expected from the Goodenough-Kanamori-Anderson rules, which
contradicts the experimental antiferromagnetic (AF) order in Ba$_2$NiIrO$_6$.
We find that the strong 2NN AF couplings are frustrated in the fcc sublattices,
and they play a major role in determining the observed AF ground state. We also
prove that the $J_{\rm eff}$ = 3/2 and $J_{\rm eff}$ = 1/2 states induced by
spin-orbit coupling, which would be manifested in low-dimensional (e.g.,
layered) iridates, are however not the case for cubic Ba$_2$NiIrO$_6$. Our PTMC
simulations show that when the long-range (2NN and 3NN) AF interactions are
included, an AF transition with $T_{\rm N}$ = 66 K would be obtained and it is
well comparable with the experimental 51 K. Meanwhile, we propose a possible
2$\times$2$\times$2 noncollinear AF structure for Ba$_2$NiIrO$_6$.
