Abstract Composite nonwovens are fabrics consisting of two or more layers that are combined by cohesion, fusion or adhesion. The z -directional tensile strength (ZTS) of such fabrics is important in various applications, including wound dressings, absorbent hygiene products, roofing carrier fabrics, absorbent wipes, filter media and wall coverings. Whilst the determination of tensile strength in the x and y directions in composite nonwoven fabrics is readily undertaken, characterisation of the resistance to delamination can be challenging, particularly when there is intensive mechanical entanglement of the constituent fibres between the layers. In this paper, the BS ISO 15754 method for determining the ZTS of paper and board has been adapted to characterise the ZTS or tensile strength in the thickness direction, of nonwoven fabrics using a universal tensile tester. The adapted method was found to be suitable for the study of ZTS in a variety of nonwoven assemblies, including mechanically bonded composite nonwovens comprised of more than one layer. Application of the modified experimental method was demonstrated following the preparation of composite nonwoven fabrics comprised of polyethylene terephthalate (PET) and glass fibre layers, mechanically-integrated by hydroentanglement. A full-factorial experiment was completed in which the main effects and interactions of water jet pressure, conveyor speed, nozzle diameter and number of injectors on the ZTS were analysed. A four-way significant interaction was observed wherein water jet pressure and conveyor speed were empirically found to influence the ZTS of the composite nonwoven fabric. The average through-thickness location at which failure of the composite nonwoven fabric occurred was characterised by a gravimetric approach and was influenced by the total specific energy and ZTS of the fabric.