Being able to stay longer is another way of helping Kiwi businesses explore new opportunities in South East Asia, Ardern said.
But she said relations with Singapore would be enhanced even more next year.
"Today's announcement marks the first step towards the launch of a broader Enhanced Partnership with Singapore that will be launched next year," she said.
That will expand co-operation between the two countries in four areas: trade and the economy, defence and security, people-to-people links, and science, technology and innovation.
Lee would be visiting New Zealand for the launch of the Enhanced Partnership and signing of the CEP.
The Closer Economic Partnership was negotiated by former Trade Minister Sir Lockwood Smith in the Bolger Government and signed in 2000.
It was the basis for expansion to Brunei and Chile in a deal known as the Trans Pacific Strategic Economic Partnership, or P4 for short.
When the four countries came to negotiate further chapters of the deal in 2008, the United States asked to join the talks, and others pile in too. That became the TPP or Trans Pacific Partnership although the US has since withdrawn.
New Zealand has four trade agreements involving Singapore: the CEP, the P4, the Asean New Zealand and Australia Free Trade Agreement (AANZFTA) and CPTPP.
It was renamed the CPTPP last year - the Comprehensive and Progressive Agreement for Trans Pacific Partnership - and it will enter into force on December 30.
The countries that have ratified are Japan, Singapore, Mexico, Canada, New Zealand, Australia and Vietnam. Still to ratify are Chile, Peru, Brunei and Malaysia.