Weather today is fine after yesterday's scrub because of deteriorating conditions and the proximity of the orbiting 450-tonne International Space Station (ISS). Bad weather also prevented launch over the weekend.
The 108m wide station orbits at 400km above Earth, about the same altitude Rocket Lab wants to reach to deploy small satellites.
A spokeswoman today said the ISS was another factor that played into when Electron could launch.
''Within our launch windows there are certain times we can't launch as we wait for satellites and spacecraft (such as the ISS) to pass. As a result, our launch window gets broken into several smaller windows where there is a clear path to orbit,'' she said.
''We had several of these clear windows yesterday and we are aware of when these will be well in advance of a launch attempt. However, as the weather began to close in it overlaid some of these clear windows, leaving us just a narrow six-minute window that was both clear of ISS movements and weather.''
So Rocket Lab chose to hold off until today.
Preparations were under way for a launch from 4pm but it could, however, be scrubbed within seconds for technical reasons or because of a shift in weather conditions.
A small crowd has gathered at the viewing site to see the launch.
The later time was a blessing for Grey Wilson, as it meant she had been able to bring her children, two-year-old Zoe, and Mack, 4 - who had been talking about rockets all day at school.
The Frasertown resident said her husband had been involved with the construction of the launch pad, but she had missed the first launch.
Rocket Lab says it will issue more updates and livestream details throughout the day. The launch will be available through nzherald.co.nz.
The New Zealand-founded company successfully reached space in May, in a global first from a private launch facility.
A communications glitch meant the flight had to be terminated before reaching orbit but Rocket Lab founder and chief executive Peter Beck has said while thrilled with the first launch, the company has learned from thousands of pieces of data.
The 17m-tall Electron orbital launch vehicle will carry two Earth-imaging Dove satellites for Planet and two Lemur-2 satellites from Spire for weather mapping and ship traffic tracking.
If needed, the company will launch a third test flight but it is keen to accelerate its transition into full commercial operations.
