The Dunedin swimmer finished in 8min 23.27sec in Paris this morning.
"I mean, that was all I had left in the tank,'' Fairweather told Sky Sport after the race.
"I'm pretty buggered after this week but I'm really proud [of] how I ended up.''
It was a battle of the swimming heavy weights with Katie Ledecky, of the United States and Ariarne Titmus, of Australia, throwing down the challenge to one another.
But Ledecky – who holds the Olympic and world record of 8min 04.79sec - stormed home in the final 200m to win gold in 8min 11.04sec.
It marked her fourth consecutive gold in the event and her second fastest time to win an Olympic gold medal.
Fairweather praised the support she received from the rest of the New Zealand swimming team over the past week.
"It’s been absolutely amazing,'' Fairweather said.
"I couldn’t do half of what I do without those guys over there. They mean the absolute world to me.
"After a bad race, after a good race, to give you a hug, to give you a pep talk, whatever you need.
"Big, big kudos to them."
Titmus won silver in 8min 12.29sec and set an Oceania and Australian national record in the process.
Paige Madden, of the United States, won bronze in 8min 13.00sec, shaving 5sec off her personal best.
Ledecky and Titmus went out hard to start the race and Titmus held her new to push Ledecky all the way.
But Ledecky pulled away with a couple of laps left and Madden put the pressure on to battle Titmus for the silver.
Fairweather sat comfortably in the middle of the pack for the majority of the race and dropped slightly at the halfway point.
It brings a close to Fairweather’s Paris Games.
She finished fourth in the women’s 400m freestyle, seventh in the women’s 200m freestyle and eighth in the women’s 4x200m relay, alongside Dunedin’s Caitlin Deans.
