We present the application of wavefront sensing to 3-dimensional particle metrology for measuring the 3-component velocity vector field in a fluid flow across a volume. The technique is based upon measuring the wavefront scattered by a tracer particle from which the 3-dimensional tracer location can be calculated. Using a temporally resolved sequence of 3-dimensional particle locations the velocity vector field is obtained. In this paper we focus on an anamophic technique to capture the data required to measure the wavefront. Data is presented from a reconstruction of the phase of the wavefront as well as from a more pragmatic approach that examines only the defocus of that wavefront. The methods are optically efficient and robust and can be applied to both coherent and incoherent light in contrast to classical interferometric methods. A focus of this paper has been the filtering techniques in order to reliably extract the particle images from the overall image field. The resolution and repeatability of the depth (or range) measurements have been quantified experimentally using a single mode fiber source representing a tracer particle. A first proof of principle experiment using this technique for 3-dimensional PIV on a sparsely seeded gas phase flow is also presented.