More than 400,000 people have pre-registered for Mighty River Power shares.
The Government plans to sell up to 49% of the state-owned company by May.
Potential investors have been able to register their interest since 5 March.
State-Owned Enterprises Minister Tony Ryall says the call centre and pre-registration website have been extremely busy since they launched.
All pre-registrations must be received by 5pm on Friday 22 March.