Advances in computational science are closely tied to developments in high-performance computing. We consider the case of shelf sea modelling where models have been growing in complexity and where model domains have been growing and grid resolutions shrinking in pace with the increasing storage capacity and computing power of high-end systems. Terascale systems are now readily available with performance levels measurable in TeraFlop/s and memories counted in TeraBytes. The scientific case is now being made for regional models at 1km resolution, allowing the accurate representation of eddies, fronts and other regions containing steep gradients. The hydrodynamic model is increasingly being coupled with other models in multidisciplinary studies e.g. ecosystem modelling and wave modelling. We show that the performance attainable from the POLCOMS hydrodynamic code is measurable at about 0.5 TeraFlop/s on an IBM p690 cluster with 1024 processors. The scalability on this system and others is excellent up to 1000 processors. We describe a wide range of optimisations which have together enabled this code to reach these performance levels.