Agricultural residues such as maize straw and chicken dung are produced in substantial quantities in Brunei Darussalam, but remain underutilised due to limited waste management and utilisation strategies. This study evaluated whether composting and co-composting these materials could produce mature and non-phytotoxic organic amendments. Accordingly, three organic amendments were produced from high-carbon and high-nitrogen agricultural wastes: (i) maize straw compost (MS), (ii) maize straw-chicken dung co-compost (MS:CD), and (iii) chicken dung compost (CD). Temperature profiles indicated differing thermal dynamics among the materials. Maize straw compost showed modest thermal activity (28.2 °C to 42.1 °C), MS:CD maintained warm-mesophilic conditions (30.4 °C to 39.0 °C), whereas CD reached a near-thermophilic peak of 50.0 °C. All three organic amendments exhibited near-neutral to slightly alkaline pH (7.19 to 7.87), C/N ratios of 11.6 to 13.9, and organic matter contents between 53.1% and 57.5%. Water-extractable aluminium ions were not detected, and available iron ions content remained low (0.012 cmol kg-1 to 0.231 cmol kg-1). Phytotoxicity assessment using a water spinach (Ipomoea aquatica F.) germination test showed no inhibitory effects for any of the organic amendments when compared with the soil-only control. Germination indices exceeded 126%, whereas vigour indices of the organic amendments were significantly higher than those observed in the control treatment. Overall, composting and co-composting maize straw and chicken dung produced mature and non-phytotoxic organic amendments, representing a promising pathway for valorising agricultural residues in Brunei Darussalam. However, further soil-based trials are required to confirm their agronomic performance.