  NaNiO$_2$ is a layered material consisting of alternating layers of NaO$_6$
and Jahn-Teller-active NiO$_6$ edge-sharing octahedra. At ambient pressure it
undergoes a broad phase transition from a monoclinic to rhombohedral structure
between $\sim$465 K and $\sim$495 K, associated with the loss of long-range
orbital ordering. In this work, we present the results of a neutron powder
diffraction study on powdered NaNiO$_2$ as a function of pressure and
temperature from ambient pressure to $\sim$5 GPa between 290 K and 490 K. The
290 K and 460 K isothermal compressions remained in the monoclinic phase up to
the maximum pressures studied, whereas the 490 K isotherm was mixed-phase
throughout. The unit-cell volume was fitted to a 2nd-order Birch-Murnaghan
equation of state, with $B=113(1)$ GPa. We observe at 490 K that the fraction
of Jahn-Teller-distorted phase increases with increasing pressure, from
67.8(6)% at 0.71(2) GPa to 80.2(9)% at 4.20(6) GPa. Using this observation, in
conjunction with neutron diffraction measurements at 490 K from 5.46(9) GPa to
0.342(13) GPa, we show that the Jahn-Teller transition temperature increases
with pressure. Our results are used to present a structural
pressure-temperature phase diagram for NaNiO$_2$. To our knowledge, this is the
first diffraction study of the effect of pressure on the Jahn-Teller transition
temperature in materials with edge-sharing Jahn-Teller-distorted octahedra, and
the first variable-pressure study on a JT-active edge-sharing nickelate.
