The Flight Centre fares from Auckland, Wellington or Christchurch are on sale until November 17 for travel between March 5 and June 19.
Flight Centre NZ managing director David Coombes said the fares were evidence of an extremely competitive market.
"This current level of competition is unprecedented and we can only advise that Kiwis take advantage of this incredibly competitive marketplace while it lasts."
He said it was unusual for Air New Zealand to offer such heavily discounted seats on the London route. He said there were "plenty" of seats in the deal.
"My interpretation is that the excess capacity is starting to impact further and wider."
Not long ago Air New Zealand fares would have been double today's deal.
It's not just airlines going toe to toe; agents also take a hit on such heavily discounted fares.
Coombes said that the relatively high New Zealand dollar versus the British pound had helped boost the number of Kiwis flying to Britain. So far this year more than 87,000 New Zealanders had made the trip.
During the last 18 months the number of airlines flying long-haul services had soared out of Auckland through the Middle East, Asia and during summer, via the United States. This was pushing down prices. While oil prices have crept up recently, they are still half of what they have been during spikes in the last 10 years and aircraft are more efficient, meaning airlines have more capability to absorb fuel-price rises.
Flight Centre's fares today are in economy rather than premium cabins, although Coombes said "watch this space."