  Employing first-principles modelling and many-body methods, the magnetic
properties of spin-1/2 chain compound NaVOPO$_4$ are explored. The extensive
first-principles calculations establish an intricate three-dimensionally
coupled model that consists of weakly alternating $J$-$J^{'}$ antiferromagnetic
chains running along cris-cross directions between two consecutive $ab$ planes,
connected via two subleading couplings, a ferromagnetic exchange along the $c$
direction ($J_c$) and a weaker antiferromagnetic exchange ($J_a$) along the
body diagonal direction. The exact diagonalization and density matrix
renormalized group study have been carried out on a two-dimensional spin model
with $J$-$J^{'}$-$J_c$ and effective $J_d$ couplings, constructed based on the
full model, for numerical ease. The $J_c$-$J_d$ phase diagram is found to host
a {\it disorder} phase with finite spin gap for comparable values of $J_c$ and
$J_d$, arising out of the competing nature of these two interactions, other
than two ordered phases. The predominant manifestation of $J_c$ and $J_d$ in
the disorder phase happens in stabilisation of a multi-magnon condensed phase,
upon gap closing by application of an external magnetic field. We further
explore the effect of tensile uniaxial strain, which is found to drive the
system from gapful to gapless ground state.
