Lawyers for the Electricity Commission say it undertook an intensive process, including consultation, before approving a planned high-voltage transmission line from south Waikato to Auckland.
The commission's decision is the subject of a judicial review in the High Court in Wellington initiated by the anti-pylon group, New Era Energy.
Earlier this week the group's lawyers said the decision was illegal and irrational.
However, the commission's lawyer, Alan Galbraith QC, said on Thursday that every issue referred to by New Era Energy was considered, analysed and determined before the final decision was made.
He said that issues raised by a dissenting commissioner who reached a view contrary to that of the majority of the Commission, relate to differences of judgment looking forward over 20 years.
Mr Galbraith said New Era Energy is simply challenging the merits of the commission's decision but that isn't something that is judicially reviewable.
The case is continuing.