A Christchurch man's pleaded guilty to fraud after running a sophisticated Ponzi scheme - masked as a foreign exchange brokerage.

Jimmie Kevin McNicholl, the director of Arena Capital Limited - which traded as BlackfortFX, has pleaded guilty to obtaining a registration as a financial services provider by deception.

The Serious Fraud Office says Arena Capital used the illegally obtained registration to give the impression BlackfortFX was a legitimate forex trading platform, but no trading was ever undertaken.

They say about 900 people invested approximately 8.3 million dollars into what was in fact - a Ponzi scheme.

Lance Jack Ryan, also known as Lance Jared Thompson, is also charged with fraud.

The men will appear for sentencing in six-weeks' time.