Transpower made a profit of $87.6 million in the six months to December, up almost 30% on the same period the previous year.
Revenue rose 11% to almost $380 million, partly due to higher transmission revenue.
Transpower chairman Wayne Brown says the surplus continues to be reinvested into the capital investment programme which includes new building projects, technology platforms, and the refurbishment and replacement of existing assets.
Mr Brown says capital expenditure in the last six months climbed by more than $100 million to $239 million.
The national grid operator will invest about $3.8 billion on the national grid, until the end of 2015.