The growing consumers’ environmental consciousness has driven competitive shippers (cargo owners) to voluntarily disclose carbon emission reduction (CER) information about their carriers (transportation service providers), aiming to establish a low-carbon image and enhance their competitiveness. Unfortunately, due to the low transparency in marine supply chains, profit-driven shippers are inclined to selectively disclose CER information. To address this, the independent audit by Environmental Non-Governmental Organizations (ENGOs) can be an effective mechanism. An analytical model is proposed to examine competitive shippers’ equilibrium information disclosure strategies, considering the asymmetry of CER information and the ENGOs’ adult. Our findings reveal the potential for an asymmetric equilibrium, where only one shipper chooses to disclose information while the other does not. We also identify two types of Prisoner’s dilemma situations when the acquiring information cost is moderate, and which dilemma can arise depending on the probability of an ENGO audit. Specifically, an increase in the probability of ENGO audits may backfire, leading shippers to reduce their CER investments and exacerbate the Prisoner’s dilemma. Our findings offer practical implications for shippers and policymakers in promoting environmental sustainability across the industry, particularly in the face of intense competition and the lack of accessible CER information.