  The highly-frustrated rare-earth based magnet Nd$_3$BWO$_9$ is a promising
candidate in the search for proximate spin liquid physics. We present a
thorough investigation on single crystals of this material using bulk and
microscopic techniques. Magnetization data reveal a fractional magnetization
plateau for three different investigated field directions. The magnetic phase
diagram is mapped out from calorimetric data and exhibits several domes of
magnetic order below 0.3 K. Propagation vectors for all ordered phases are
presented. The results suggest complex ordering in this material, and unveil
the existence of a commensuration transition of the propagation vector at zero
magnetic field. A scenario where interplane exchange interactions are essential
to a magnetic model of Nd$_3$BWO$_9$ is discussed.
