A Christchurch man has been sent to jail for nearly two and a half years after being convicted of 45 child sex abuse charges.

Scott Johnston has been sentenced at the Christchurch District Court after pleading guilty to 25 charges of distributing and possessing child sex abuse material, and 20 of possessing objectionable publications.

Microsoft and the United States Centre for Missing and Exploited Children alerted Internal Affairs to Johnston’s offending in 2012.

An Internal Affairs covert operation tracked the 47-year-old's behaviour and police were able to locate him and seized his computer equipment and mobile phones.

A total of nearly 24,000 files were found.