The paper presents a reasonable choice of simplified models of steady and unsteady flows in medium-and high-pressure gas pipelines. An effective algorithm is proposed that makes it possible to quickly calculate the parameters of an unsteady flow regime caused by the occurrence of a low-or medium-intensity gas leak. The proposed models and calculation algorithm enable the rapid detection of gas leaks using model-based methods for many real-world gas pipelines. Sample calculations are provided for the distributions of gas-mixture flow rate at various times for low-and medium-intensity leaks occurring at different locations along the medium-and high-pressure gas pipelines.