The NZ Climate Science Coalition is made up of people who are sceptical about global warming and share concerns about "misleading information" on climate change, according to the website.
The disputed data had high importance as it was used in policy making and to predict future trends, said Mr Sissons.
The data relates to adjustments made to readings from the "Seven-station Series" (7SS) - stations in Auckland, Masterton, Wellington, Nelson, Hokitika, Lincoln and Dunedin - used by Niwa for national temperature records.
They showed national temperatures had risen almost 1 degree Celsius in the past century, which was almost 50 per cent above the global average for the period, said the trust.
Instead of using internationally-accepted methodology, Niwa allegedly used an untested and unscientific method by former Niwa employee James Salinger.
'If you apply the method correctly then what you have is a clear error on the part of Niwa for not using the recognised scientific method. It's potentially a very large consequence," said Mr Sissons.
Temperatures recorded at stations were moved half a degree downwards on average in early readings. This had created an inaccurate overall upwards rise temperature in the last century, he said.
"Is that valid that you can expose a trend, simply by pushing down the readings in the early part to make them colder? Then when you compare them to the modern ones you get a steep increase."
A review of the data using a scientific method carried out by the trust highlighted the inaccuracies, said Mr Sissons.
"If they had followed the correct method they should have come up with a different result."
Temperature spikes could be attributed to factors such as urbanisation, rather than climate change, he said.
NIWA has said it is confident in its science and the integrity of its work.
The defence is due to begin its evidence on Thursday.
