A number of environmental groups have joined forces in High Court action against the Electoral Commission.
It follows a Commission ruling that the "Climate Voter" initiative launched by Greenpeace, Forest and Bird and other environmental groups falls under the umbrella of election advertising.
The ruling means the initiative would be subject to election advertising rules relating to wording used on campaign material and limits on advertising spending.
The environmental groups' lawyer, Dr Matthew Palmer told the High Court in Wellington his clients recognise the need to create a level playing field for spending by political parties but that should not extend to restricting free speech in advertisements which do not advocate for a particular party or candidate.
Dr Palmer said the current law was intended to avoid a third party campaign such as that run by the Exclusive Brethren before the 2005 election, but it does not cover groups advocating for specific issues.
A lawyer for the Electoral Commission Peter Gunn told the court there must be a level playing-field when it came to election spending.
Mr Gunn said the suggestion the material published by the groups was non-partisan was inconsistent with the legal test of what constituted election advertising.
He said the test was whether a reasonable observer would regard a publication as encouraging voters to vote or not vote for a certain party or candidate.