The employment rate rose to 68.3 per cent, the highest rate since the series began more than 30 years ago.
Speaking to media at the Rātana celebrations, Ardern said she was "incredibly proud" of the drop in the jobless figure."
She said that 3.9 per cent number "will fluctuate, but [we're] very pleased with the progress we have made".
"To have unemployment the lowest it's been in a decade and employment the highest it's been in three is incredible."
She was particularly pleased to see Māori unemployment come down.
Finance Minister Grant Robertson said this was an "excellent result".
He said from time to time, unemployment numbers do fluctuate, but today's figures from Stats NZ show the economy is working.
"Jobs are being created and unemployment is dropping, while employment is increasing."
He said there were various things in the economy driving the lower unemployment numbers, including growth in New Zealand's export and services sector.
Robertson also said some of the Government's policies had helped too.
"We're working hard to make sure we have active labour market policies underway,"
including industry partnerships with the Ministry of Social Development and the Mana and Mahi programme for getting young people into work, he said.
But National's Finance Spokeswoman Amy Adams said surveys show some firms are finding it difficult to find skilled workers.
The most recent NZIER quarterly survey of business opinion showed this was one of the main areas of concern for businesses.
Robertson said businesses are constantly highlighting to the Government issues they have around finding skilled staff.
"But I don't think we should let that get in the way of a very good result – we want to be creating jobs, we want to see unemployment going down."
He said it is now the Government's job to work alongside businesses to ensure there is enough skilled staff in the market.