The seed coat of the legume lupin which is rich in insoluble dietary fibre is a major by-product in human food applications. Extrusion cooking has been demonstrated to increase desirable soluble dietary fibre in the Australian sweet lupin seed coat. In this study, processing condition of twin-screw extrusion cooking was optimised using a central composite rotatable design to increase soluble dietary fibre in lupin seed coat from 44.17 g/kg up to 113.69 g/kg dry basis. The high levels of polyphenols in the seed coat were retained. The optimal extrusion conditions which achieved maximum levels of soluble dietary fibre, total free phenolic content and total free individual phenolic content simultaneously were identified and validated. The extrusion cooking largely had no or slight effects on bioaccessibility and bioavailability of the selected minerals and individual polyphenols. The extrusion cooked lupin seed coat could be a natural antioxidant dietary fibre source for human consumption. •An extrusion processing was optimised using multi-response surface design.•Extrusion maximised soluble dietary fibre content in Australian sweet lupin seed coat.•High levels of polyphenols in the seed coat were retained after extrusion.•Extrusion showed no effects on in vitro bioavailability of minerals and polyphenols.