[Grandmother] faces 10 years in jail for a fake letter ‘canceling’ an oil industry conference

Is that in the spirit of the law? I could see a fine, scaled to personal wealth. But imprisoning them, what threat did she actually pose? What were the worst possible consequences of massively profitable companies not talking for a bit?

To be facing 10 years she must have been charged under section 255 subsection 1 of the Crimes Act 1961. It doesn't seem from the article that her intentions met the criteria in that subsection which states "Every one is liable to imprisonment for a term not exceeding 10 years who makes a false document with the intention of using it to obtain any property, privilege, service, pecuniary advantage, benefit, or valuable consideration."

New Zealand, you disappoint me. Looks like the judges got owned by the oil industry