Ms Armstrong, of Lake Hāwea, reached the halfway point about 3pm, in seven and a-half hours.
The official length of the lake is 39.4km in a straight line, but because she strayed from the line along the way, she ended up swimming closer to 40.1km in distance.
She finished just before 10pm, with an unofficial time of 14 hours and 24 minutes, making her the first person to complete the marathon swim of the lake.
She began at 7.30am, at the north end of the lake, near where the Hunter River enters Lake Hāwea.
Ms Armstrong, an architectural designer, has competed in many open water swim events in recent years, and was second woman home in the Ruby Swim 5km last month (non-wetsuit category).
More recently, she switched her sights to more temperate open water goals.
Lake Hāwea sits in a range of between 12°C and 18°C.
Ms Armstrong was swimming according to marathon swimming rules, which prohibit wetsuits, intentional contact with boats or support personnel, and drafting.
Her support team used two boats: a small inflatable for navigation and feeding, and a larger safety boat.
Before her swim, Ms Armstrong posted on social media: "Am I nervous? Absolutely. Am I excited? Heck yeah!"
