Invasive rats can be capable swimmers, able to cross substantial water channels of hundreds of metres to colonise islands. This dispersal capability puts at risk islands close enough to infested areas for rats to reach unassisted. When reinvasion rates are high, biosecurity surveillance on islands might be supported by source population control to prevent re-establishment. However, biosecurity surveillance can only detect reinvading rats when they arrive and the source of reinvading rats might remain unknown. In order to validate hypothesised source sites we used the biomarker Rhodamine B to detect water crossings by ship rats (Rattus rattus) from Great Barrier Island to two neighbouring island groups where eradications had been attempted in the past but were subject to high levels of reinvasion. We detected Rhodamine B in rats dispersing to both island sites, confirming the dispersal of rats across the water gaps, and confirming the hypothesised source sites where additional rat control may be warranted. Chemical markers provide a cost-effective tool to label individuals and determine point sources of origin, most powerfully in situations where dispersal is happening over a scale of weeks.