The results of existing constitutive models describing the behaviour of metal powder during compaction processes are very different. This reveals the high sensitivity of the mechanical behaviour of porous materials to the shape, arrangement and distribution of particles and pores. In order to clarify these discrepancies, the compaction behaviour under hydrostatic loads and high temperatures for a Nickel-based superalloy has been characterized. For this characterization, a numerical optimization procedure has been defined. In parallel, finite element models at a mesoscopic level have been built with the aim of estimating the parameters that define the mechanical behaviour of metallic powder under hydrostatic loads. Then a homogenization procedure has been used to compute the macroscopic behaviour of the powder. The results of both the experimental characterization and the mesoscopic models emphasize the limits of the analysed literature constitutive models to reproduce the compaction behaviour of the considered Astroloy powder.