"I believe that it was the biggest ever capital raising in New Zealand," ANZ strategist David Croy said.
"It is certainty the biggest ever bond issue that we have ever seen here," he said.
Market strategists said the successful issue would take a bit of the funding pressure off next year.
The Treasury had initially specified a minimum target amount of $2b but did not set a cap.
The amount raised reflected the high level of interest so the deal was "upsized", Croy said.
Bloomberg said results echoed similar sales in other developed markets, with Australia also raising a record amount of funds recently.
Westpac senior markets strategist Imre Speizer said investor appetite was strong, aided in part by the relative stability of the domestic interest rate markets, given the Reserve Bank's commitment to keep its official cash rate on hold until next year and its bond buying programme.
Hamish Pepper, fixed income and currency strategist at Harbour Asset Management, said the successful issue reflected the fact that rates were close to zero in many parts of the world, due to widespread quantitative easing by central banks.
The previous record for a syndicated New Zealand bond sale was $3.5b, set at April's auction of May 2031.
The sale comes after the Reserve Bank almost doubled its quantitative easing programme to $60b, which is aimed at keeping a lid on rates while the economy recovers from the effects of the Covid-19 pandemic.