The individual morphology and size distribution of nano-cavities in a necked tensile specimen made of semicrystalline high-density polyethylene (HDPE) have been observed and quantified in three dimensions using magnified synchrotron radiation holotomography. Here, using a voxel size of 59.7 nm, permanent voids in the unloaded state after yielding are observed that can only be assessed in averaged manner at finer length scales using small angle X-ray scattering (SAXS) techniques. Observation of the same sample using typical synchrotron radiation tomography with micron resolution is also given highlighting the novel opportunities arising with magnified synchrotron radiation holotomography in comparison. Two populations of voids elongated in loading direction are observed: A population of large cavities that has nucleated around inclusions and a population of small cavities. All cavities are equiaxed in radial directions and elongated in loading direction which is consistent with SAXS findings of other studies. Void volume fraction, void size distributions and characteristic dimensions of the voids are assessed using 3D image analysis.