Solid Energy's half-year profit increased by 56% but the company warned of significantly lower profits in the next six month period.
The state-owned coal miner made $70.3 million in the six months to December, compared with $45.2 million in the same period a year earlier.
It sold a record 2.4 million tonnes of coal in the six month period - a 9% increase on the same time a year earlier - at a time when prices were strong and production was going well at its open-cast mine in Stockton.
The firm's coal exports rose 6% and domestic coal sales rose 12%.
The result was boosted by three shipments of coal carried over from the previous year because of damage to port of Lyttelton in the June earthquakes.
Revenue rose by a quarter to $538 million.
However Solid Energy chairman John Palmer says profits in the second half of the financial year are likely to be significantly lower.
This is due to the strengthened New Zealand dollar and a fall in coal prices because of weakened Chinese demand for coking coal.
The Government plans to sell up to 49% of the company and three other state-owned energy firms.