An ineffective submerged orifice fishway was retrofitted with a Deelder lock and assessed for fish passage over two consecutive years (2002–2003) at Balranald Weir on the Murrumbidgee River, Australia. The assessment, using paired entrance and exit sampling, over three different cycle times, yielded a total of 13 448 fish (11 species) and 859 freshwater prawns (one species) passing through the lock. Australian smelt Retropinna semoni Weber, bony herring Nematolosa erebi Günther and western carp gudgeon Hypseleotris spp contributed 89% of the total catch. The maximum number of fish caught in a single day was 854. Cycle time did not have a significant effect on the abundance or composition of fish using the fishlock and sizes of fish sampled from the exit trap ranged from 12 to 540 mm fork length. Length–frequency analysis also demonstrated that all size classes of the most common species were able to negotiate the lock successfully. Movement of fish varied significantly with time of the day. The greatest movement occurred between 1200 and 1600 and few fish migrated at dawn or at night. The Deelder lock improved passage for a much wider range of species and size classes than within the existing submerged orifice designs in other areas of the Murray-Darling Basin. The design is therefore recommended for installation at other sites, especially where small-fish passage is a priority. Copyright © 2007 John Wiley & Sons, Ltd.