Using a multi-disciplinary approach, this study quantifies horticultural postharvest losses of two medium-sized (annual pack volume 4500 t) commercial, domestic, tomato supply chains. Quantification of loss was based on weight or volume, consistent with direct measurement methods of the Food Loss and Waste Accounting and Reporting Standard 2016 and qualitative techniques were used to identify the drivers of the loss and contextualise the findings. Postharvest loss was found to be between 40.3% (55.34 t) and 55.9% (29.61 t) of the total harvestable product. It was determined that between 68.6% and 86.7% of undamaged, edible, harvested tomatoes were rejected as outgrades and consequently discarded due to product specifications. Between 71.2% and 84.1% of produced tomatoes were left in the field and not harvested. This study highlights significant factors contributing to high levels of food loss and waste. Edible products are being removed from the commercial food supply chain, rejected as outgrades deemed cosmetically defective due to market-based decisions. With only 44.1% and 59.7% of the harvestable crop reaching the consumers of the two supply chains, respectively, it is perhaps more appropriate to describe a food "waste" chain as opposed to a food "supply" chain.