Governments aim to mitigate natural hazards' impacts through a disaster early warning system. Drawing on citizen co-production theory and resilient information infrastructures framework, we empirically examined government use of Twitter Tsunami Early Warning Civic Network and citizens' roles in co-providing timely and actionable information. The Indonesian government issued its tsunami early warning Tweet, which was “re-tweeted” without delay by its followers to their own followers to warn tsunami hazards during the 2012 earthquake. Within 15 min it reached over 4 million Twitter users. Based on our case study and social network analysis of Twitter information flows and exchanges within the network, we found that the speed and enormous reach of the government's Twitter tsunami early warnings would be significantly less without citizens' direct participation in re-tweeting, hence influencing greater control of the network. We present evidence for net-savvy citizens' co-production effects on increased government efficiency in providing time-critical public information services.