Long-term use of urinary catheters in spinal cord injury (SCI) patients often results in complications such as constant colonisation by polymicrobial biofilms and frequent catheter-associated urinary tract infections (CAUTIs). In this study two distinct multi-species biofilm models derived from biofilms collected from clinical catheters used by patients with SCI were evaluated for their ability to inhibit colonisation by a new invasive bacterium. Model C-1 consists of Escherichia coli, Klebsiella pneumoniae and Streptococcus agalactiae while model C-5 consists of K. pneumoniae, Enterococcus faecalis, Pseudomonas aeruginosa and E. coli. Here, P. aeruginosa PAO1 and Staphylococcus aureus, an isolate from a CAUTI episode, were introduced into the multi-species biofilms that had formed over a 24-hour period under flow conditions. After an additional 3 days incubation colony-forming unit (CFU) counts were used to determine the biomass of each species within the polymicrobial biofilms. The results demonstrated that the invading bacteria were able to colonise the urinary catheter at a much lower rate than when grown as mono-species biofilm without the established biofilm. The invading bacteria could not take over the biofilms, and the total biomass of pre-existing bacteria within the biofilm models remained unaffected by their presence. This research offers valuable insights into the protective mechanisms of pre-existing biofilms, suggesting potential avenues for new policies in CAUTI treatments as well as developing novel treatment strategies. Further investigation could uncover novel pathways for effectively managing CAUTI during long-term catheterisation.