Determining nutrients in seawater with accuracy is crucial for Oceanographical measurements. Since 2011, Commonwealth Scientific Industrial Research Organisation (CSIRO) has used the certified reference materials for nutrients in seawater (RMNS) to determine analytical accuracy for nitrate, nitrite, silicate, and phosphate. However, ammonium, a biologically important macro-nutrient, has been absent from the RMNS. To address this, CSIRO created a bulk quality control (BQC) containing the 5 macro-nutrients. This was done by spiking nutrients into filtered seawater, collected from oligotrophic oceans, preserving with mercuric chloride, and subsampled into 400 polypropylene tubes. The BQC, was optimised for coastal and deep ocean concentrations and remained stable for all 5 nutrients over 9 months, when the batch was depleted. In 2022, the BQC was used in an informal Proficiency Trial (PT) involving 8 laboratories. Custom software was developed applying statistical methods following ISO 13528, "Statistical methods for use in proficiency testing by interlaboratory comparison," to derive metrics such as the Robust Average, Assigned Value, z-scores, and EnScores. The PT gave insight into the potential use as a reference material. Methodology and software are being expanded to further characterise stability and homogeneity. The BQC offers a cost-effective reference material with optimal nutrient concentrations including ammonium.