Klebsiella pneumoniae that coproduce New Delhi metallo-β-lactamases and serine β-lactamases are of particular concern due to limited treatment options. Aztreonam/avibactam is a novel β-lactam/β-lactamase inhibitor combination used against difficult-to-treat infections, however some resistance towards this combination has already been observed. Therefore, the aim of this study was to investigate the synergistic potential of aztreonam/avibactam in combination with colistin. 48-hour dynamic time-kill experiments with aztreonam/avibactam and colistin were assessed to evaluate the in vitro activity of the drugs alone and in the triple combination, using the Hollow Fiber Infection Model. This allowed the drug concentrations to closely mimic those observed in plasma of critically ill patients receiving standard dosing. Population analysis profiling was performed at 0, 12, 24 and 48 hours by plating the samples on agar containing 4x and 8x MIC of aztreonam/avibactam and colistin, to investigate emergence of resistance. Colistin alone caused rapid initial killing (4.7 log10 CFU/mL reduction) followed by regrowth after 3 hours, whereas aztreonam/avibactam alone exhibited slower killing, showing bactericidal effect after 4.5 hours. The triple combination demonstrated 4.3 log10 CFU/mL reduction after 30 minutes, and this bactericidal effect was sustained over the 48-hour experiment. Bacterial growth was observed on 4x and 8x MIC colistin plates after colistin single treatment, but not after treatment with the combination. The combination was overall more effective at bacterial eradication compared with either monotherapy, and no emergence of resistance was observed. However, these results are based on limited in vitro data, and more research is needed for further validation.