Todd Energy says it is determined to prove that the price of natural gas is being kept higher than it needs to be, because of constrained production.
The company has a court case scheduled for next February against Shell and another energy company, OMV, over production levels at the Pohokura gas field in Taranaki.
A preliminary ruling has been lost in the Court of Appeal, over the release of documents to prove its case.
But Todd Energy says it is determined to press on.
It alleges Shell and OMV, who are its partners, are producing 20% less gas from Pohokura each year than they are able to, which substantially reduces competition in the market and holds prices artificially high.
Todd says the reduced production levels were ordered because Shell and OMV can outvote it due to their combined ownership of the gasfield being greater.
It says the loss cost it money and is seeking damages.
Since the wind down of the Maui field was announced, Pohokura has become New Zealand's most important source of gas.
Both Pohokura and Maui have been subject to repeated lawsuits involving Todd and its partners.
