  We set forth the structural and magnetic properties of the frustrated
spin-$5/2$ triangle lattice antiferromagnet Na$_3$Fe(PO$_4$)$_2$ examined via
x-ray diffraction, magnetization, heat capacity, and neutron diffraction
measurements on the polycrystalline sample. No structural distortion was
detected from the temperature-dependant x-ray diffraction down to 12.5 K,
except a systematic lattice contraction. The magnetic susceptibility at high
temperatures agrees well with the high-temperature series expansion for a
spin-$5/2$ isotropic triangular lattice antiferromagnet with an average
exchange coupling of $J/k_{\rm B} \simeq 1.8$ K rather than a one-dimensional
spin-$5/2$ chain model. This value of the exchange coupling is consistently
reproduced by the saturation field of the pulse field magnetization data. It
undergoes a magnetic long-range-order at $T_{\rm N} \simeq 10.4$ K. Neutron
diffraction experiments elucidate a collinear antiferromagnetic ordering below
$T_{\rm N}$ with the propagation vector $k = (1,0,0)$. An intermediate value of
frustration ratio ($f \simeq 3.6$) reflects moderate frustration in the
compound which is corroborated by a reduced ordered magnetic moment of $\sim
1.52$ $\mu_{\rm B}$ at 1.6 K, compared to its classical value ($5 \mu_{\rm
B}$). Magnetic isotherms exhibit a change of slope envisaging a field induced
spin-flop transition at $H_{\rm SF}\simeq3.2$ T. The magnetic field vs
temperature phase diagram clearly unfold three distinct phase regimes,
reminiscent of a frustrated magnet with in-plane (XY-type) anisotropy.
