Kelp forests have received increasing attention due to widespread declines in many regions, alongside stability or expansions in others, and the potential to reverse loss where it has occurred. Such reversal requires understanding which techniques of intervention can restore their coverage at large scales in cost-effective ways. We investigated two techniques of kelp forest restoration along a metropolitan coast in southern Australia: ‘passive’ restoration, involving seascape-scale improvements to water quality for multi-species restoration of kelp forests and seagrass meadows, and ‘active’ restoration, which involves local-scale kelp transplants to restore kelp forests. Passive restoration through water quality improvements (i.e., wastewater treatment plant upgrades) over 10 years yielded kelp recovery of 1.15 hectares. Active restoration through transplanting local 1 m² patches of kelp over 3 years yielded 0.0008 hectares. While each technique produced next-generation recruits to sustain canopies, the annual rate of kelp expansion was ~386 times greater via passive restoration than active restoration. The passive approach required considerable initial investments and political will; however, the cost per hectare (~USD$ 42,250 ha ^-1) is substantially lower compared to the active approach of transplanting kelp (~USD$ 292,915 ha ^-1), which appears to be a riskier process with lower returns on financial investment. Passive restoration through water quality improvements facilitates substantial kelp gains, offers increased scalability, and provides broader ecosystem benefits across seascape scales, though the initial investment required is substantial. Active restoration may provide immediate canopies of kelp through transplantation, but provides a negligible increase in kelp cover over time. Funding for upscaling remains a significant limitation. The effectiveness of each restoration technique depends on the local coastal ecology, with passive methods offering benefits at broader spatial scales and active methods providing more immediate results. However, sustained and large-scale recovery of kelp ecosystems appears to require thoughtful approaches that directly address the underlying drivers of coastal degradation.