Top Energy chief executive Russell Shaw said the proposal could make the Far North self-reliant in electricity.
That would mean not only a more reliable power supply, because the district would not depend on transmission across Auckland from the Waikato, but it could also save Far North households money.
Currently Top Energy is fighting a proposal by the Electricity Authority to allow Transpower, which operates the national grid, to bump up lines charges in Northland by close to $300, or 172 per cent, per household.
At the same time big users like the aluminium smelter at Bluff would see lines charges drop by as much as $50 million a year.
However, Mr Shaw said if the Far North exported rather than imported power, the generator - in this case Top Energy - would have to pay the increased transmission charges, not the consumer.
If the expansion went ahead the availability of cheap, locally produced power and steam for heating could allow the development of an industrial park creating up to 200 jobs.
A similar scheme had seen a timber processing plant and Miraka, a Maori-owned dairy factory, set up next to a geothermal power station on the Central Plateau.
Mr Shaw said care had been taken in planning the expansion to ensure temperatures and pressures at Ngawha's thermal pools would be maintained. To date the pools, which are culturally and historically significant, had not been affected.
Even if the expansion went ahead, modelling showed just one percent of the available energy would be used.
Top Energy is likely to use the same Israeli firm that designed the first two plants, though that will depend on price because it is no longer the only producer of power plants suitable for Ngawha's conditions.
The commissioners conducting the hearing are expected to take a month to reach a decision. If the consents are granted they can be appealed to the Environment Court.
The consent process so far has cost Top Energy $3 million. An appeal would add another $1m.
The company is also applying to renew its consents for the two existing plants.
Peak power demand in the Whangarei-Kaipara area served by Northpower is 170MW. Whangarei would be able to use the Far North's surplus power but would still depend on electricity generated further south.
Ngawha's geothermal field is the biggest in the country outside the Central Plateau.
