New monitoring system for purpose free release of materials into the living environment with a 600 L container counting geometry was designed and developed. Considering that monitoring system is classified as a dedicated measuring instrument the metrological certification is necessary. For purpose of metrological qualification tests the checking verification container (phantom) was proposed and manufactured. The arrangement of the reference sources in the volume of checking container was designed so that it approximates evenly activity distribution considered at efficiency calibration. Description of monitoring system, measurement uncertainty assessment, design of checking container and results of metrological qualification tests are in more detail discussed. Highlights • The procedure for metrological certification of the gamma spectrometry monitoring system based on the twin HPGe detectors with 600 L rectangular container counting geometry was proposed. • The checking container (phantom) containing 24 holes for rod calibration sources evenly mounted through the container in direction of detectors axes was designed and manufactured. • Proposed arranged of holes for rod sources approximate the homogenous distribution of the activity in container volume that is expected during usual measurement.