Civil Defence is continuing to monitor the Waikato River after it reached the critical 15 metre level overnight.
Water is being released to relieve the pressure on Lake Karapiro. The release began at 6pm. On Thursday morning, the level was at 15.15 - 15.20 metres.
Civil Defence says there is a risk houses could be marooned if the river peaks at 16 metres.
It says there is no immediate concern for people's safety, but it's put 30 homes on Ann Street and Awatere Avenue on alert. Waikato Stadium is on standby in case there's an evacuation.