Mr Key said the deal could be sealed within a year.
"Now they're turning their mind to it, I think they will, with some urgency make the decision they're either going to move ahead or not.''
Talks scheduled for late this year, possibly October "will tell us whether they're actually really serious and what's on the table''.
Mr Key told the business audience South Korea had completed major free trade agreements with the United States, the European Union and Chile and was negotiating others.
"We need to get our deal done to ensure that the bilateral trade relationship keeps pace with the trade relationships we have with other countries.''
The lack of an FTA between the two countries was ``hurting'' New Zealand companies due to the preferential treatment received by rivals from other nations which had concluded agreements.
That disadvantage was likely to grow.
"As every day goes past we're in a worse position.''
With South Korean firms paying just $5 million in tariffs on exports to New Zealand, while Kiwi firms were paying $200 million on goods going the other way, most of the upside to an FTA appears to be in New Zealand's favour.
But Mr Key highlighted the potential for co-operation between New Zealand and South Korean businesses on joint ventures, offering Zespri's South Korean growing operations as an example.
Addressing the South Korean agriculture sector's concerns, Mr Key emphasised the two countries did not compete in most sensitive agricultural sectors like rice.
"Many of our products compete primarily with other imported products, so we do not see ourselves as a threat to Korean farmers.''
An FTA would also help South Korea address the challenges associated with food and resource security.
Today's Kiwi Chamber event also showcased robots and folding electric bike the YikeBike as examples of how South Korea and New Zealand could work together on high tech deals rather than just on primary products.