<p>We present a case study on the effects of a commercial urea spill on a stand of <i>Avicennia marina</i> subsp. <i>australasica</i> mangroves (grey mangrove) in South East Queensland, Australia, from data collected during an investigation into environmental harm. We describe the localised impact of the spill and analyse nitrogen stable isotopes and ammonium concentrations from the spill source, soil and the surrounding vegetation. The spill source (ultrapure urea) had <i>δ</i>¹⁵N values of −2.93‰, and <i>δ</i>¹⁵N values in grey mangrove leaves increased significantly with distance from the spill, indicating the decreasing uptake of spill-derived nitrogen with distance, with mean leaf <i>δ</i>¹⁵N −4.28‰ near the spill source compared to 4.57‰ in control samples. Mean <i>δ</i>¹⁵N of mangrove roots at the impacted area was 2.97‰, rising to 7.96‰ at the control area. We found lower <i>δ</i>¹⁵N in leaves compared to roots and spilled urea <i>δ</i>¹⁵N values which support previously described nitrogen pathways that have shown that ammonium assimilation occurs mainly in roots, while nitrate assimilation happens in both roots and shoots. Lower <i>δ</i>¹⁵N in foliage compared to roots was consistent with other studies on grey mangroves showing leaf <i>δ</i>¹⁵N values significantly lower than root material. Linear regression revealed a significant correlation between sediment pH and ammonium concentration, with pH reaching 9.00 in the most contaminated samples, which had ammonium concentrations in sediment of 2490 mg kg⁻¹. Our findings highlight the localised yet significant impact of urea spills on mangrove ecosystems and support a theoretical framework for using stable isotope analysis in conjunction with sediment ammonium concentrations when assessing these impacts.</p>