A man who dodged $4 million in duty and GST charges for importing the alcoholic Korean drink Soju has been sent to jail for three-and-a-half years.

Jun Lee, 36, the director of Auckland-based company Youngs NZ International Limited, was convicted for using forged documents and evading the correct payment of duty and GST for 50 containers of Soju that were imported from South Korea over a four-year period.

Customs began investigating the company in 2014, and identified that Lee was using double invoices to show a lower value and declaring shipments as beer instead of Soju, as beer has a lower duty rate.

Lee imported more than a million bottles into New Zealand in the 50 shipments.

Customs investigations manager Bruce Berry says the sentencing in the Auckland District Court should send a strong message that Customs takes revenue fraud very seriously.

"Mis-declaring, concealing goods or using under-valued invoices is a serious offence that Customs can investigate," he said.

"We may not take immediate steps as we gather evidence of systematic fraud, but perpetrators can expect to be prosecuted."