The ability to accurately and precisely measure dissolved gas concentrations in groundwater is crucial for environmental monitoring purposes. Unfortunately, the collection of dissolved gas samples in groundwater is challenging because of the loss of dissolved gas in the groundwater as a result of inadequate sampling methodologies and the need to avoid exposure of the samples to the atmosphere. In environments where water can contain large amounts of methane and carbon dioxide (e.g., coal basins), there is a high probability of separation between the water and gas phases during sample collection, making it impossible to accurately measure the gas concentrations in a groundwater sample. In this paper, we describe the design and development of a novel downhole sampler that is used to collect dissolved gas samples in groundwater at in situ pressure. The sampler was used to collect samples from 26 bores in a proposed coal seam gas field in regional New South Wales, Australia. Comparison of CH4 concentration data showed that the gas concentrations were higher using our methodology than when using alternate methodologies, especially when CH4 concentrations were >10000 μg L-1. Not surprisingly, results from the new methodology suggest that isotopic fractionation occurs when samples are collected using alternate methodologies.