From Wairakei Drive (SH5) Wairakei Road is signposted, beside the Wairakei Terraces & Thermal Health Spa. A signpost on the corner has LOOKOUT on it (and also NO EXIT).
Take note of the warning to turn car lights on because of the steam coming from the Spa - it really can totally block all visibility.
The road up to the lookout is 2.5km and is an interesting drive beside massive pipes and volumes of steam. The road even passes under pipes forming an upside down U shape. A noticeboard at the top has explanations about the project.