Purified Recycled Water (PRW) is a valuable water source to supplement existing drinking water supplies, but few studies have investigated its introduction to lakes and the ‘barrier effect’ of diluting PRW with lake water. Here we evaluate whether Lake Wivenhoe (Somerset Region, Queensland), the largest drinking water storage in South East Queensland, could act as a barrier to reduce risks to public health in the highly unlikely event of introduction of pathogens in PRW entering the lake caused by failure of treatment processes. This study helps to inform regulators and water utility stakeholders of the effectiveness of lakes as a nature-based barrier that supports considerations of indirect potable reuse for water security.