Warren Gatland’s 124th match in charge of Wales ended in defeat against his native New Zealand, but he finished his 12 years considerably in credit and said he would be heartbroken if the gains made since he took over, at a time when they were 10th in the world rankings, are squandered in the coming seasons.

Gatland, who is taking charge of the Chiefs in New Zealand next year before having a sabbatical to prepare for the 2021 Lions tour to South Africa, will be back in Cardiff at the end of the month. He will be coaching the Barbarians against Wales who will be under his successor as head coach, Wayne Pivac, for the first time.

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“I hope they continue to build on what we have achieved in the last 12 years,” he said after Wales’s defeat by New Zealand in the bronze medal match meant they finished fourth in the tournament for the second time since he took over. “It would break my heart if Wales went back into the doldrums.

“I will miss the energy of the players, who are a great bunch of men to work with. Everywhere we have been in Japan the reaction of how they have conducted themselves has been incredibly positive. It was disappointing to lose my last match but it was clear that the five-day turnaround after the semi-final and the loss of four players took a toll.

“I just hope that Wales continue to be competitive against the top nations. It has been good for Stephen Jones [Pivac’s attack coach] to be here and see how we do things. The opportunity for the next group is to come in and improve on what we have created. I am looking forward to the Barbarians game because it will give me an opportunity to say thank you to the Welsh public and show how much I have enjoyed my time there.”

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Gatland’s New Zealand opposite number, Steve Hansen, left on a winning note and said he expected the All Blacks to continue to be a force in the game, although he warned reporters: “I have promised that I will not be talking about what they are doing and what they should be doing. If anyone rings to ask me, it won’t be a very good conversation.”

Hansen was asked whether he welcomed Gatland’s return to New Zealand, but gave an equivocal reply, saying: “He has done a wonderful job with Wales, but I am not sure how it will work in New Zealand because he is only spending a year with the Chiefs and doing the Lions after that. There does not seem to be a lot of continuity, but I am sure they will work their way through that.”

• This article was amended on 4 November 2019. The 2021 Lions tour is to South Africa, not New Zealand as stated in an earlier version.