Josh Giddey's Oklahoma City Thunder have clinched the top seed in the NBA's Western Conference playoffs with a 135-86 battering of depleted Dallas Mavericks.

The Thunder ended the regular season with the same 57-25 record as defending champions Denver, but own their head-to-head tiebreaker.

They entered the final day of the regular season on Sunday in a three-way tie with the Nuggets and Minnesota - the first time in NBA history three teams entered the final round sharing a conference lead.

Giddey (eight points, eight assists, six rebounds) continued a rich vein of form for OKC, while All-Star teammate Shai Gilgeous-Alexander scored 15 points in one half of action.

The Thunder will have to wait for the Play-In Tournament's conclusion to learn their first-round opponent.

"Nothing's guaranteed ... so, to be a post-season team, to be a one seed - we don't take that for granted at all," Thunder coach Mark Daigneault said.

"We're incredibly excited. Now it's just a matter of letting it rip with that opportunity."

The Mavericks, including Australians Dante Exum and Josh Green, had already clinched the West's No.5 seed and have a playoffs first-round matchup with the Los Angeles Clippers.

Green started and had four points and three assists for the Mavs while Exum sat out the match along with superstars Luka Doncic and Kyrie Irving.

The Nuggets claimed the second seed in the West with a 126-111 win at Memphis.

In Minneapolis, the Timberwolves lost 125-106 to Phoenix and had to settle for the West's third spot - and a rematch with the Suns in the playoffs first round.

Australian Joe Ingles is headed back to the playoffs after his Orlando Magic beat Milwaukee 113-88 to finish fifth in the Eastern Conference.

Ingles had three points and two assists from the bench as Paolo Bandero led the Magic with 26 points and 11 rebounds.

The Bucks played again without Giannis Antetokounmpo, who has a lower leg strain that's left his availability for the playoffs in question.

Young Australian Dyson Daniels' New Orleans were bumped out of a coveted top-six seeding in the West, beaten 124-108 by the LeBron James-inspired Los Angeles Lakers.

James had 28 points, 17 assists and 11 rebounds as the Lakers claimed the eighth seeding, setting up a rematch with the Pelicans in the conference play-in.

In Boston, the league-topping Celtics rested their starting line up but still claimed a 132-122 victory over Washington.

The Celtics (64-18) are top seeds in the East after reaching 60 or more wins for the 15th season in club history - last time they hit the mark was in 2008, when they captured the NBA title.

In New York, Jalen Brunson capped a memorable regular season with his 11th 40-point game, carrying the Knicks to the No.2 seed in the East with a 120-119 overtime victory over Chicago.

The Knicks finished 50-32 and moved past Milwaukee, who will face the Indiana Pacers in the playoffs.

The Pacers beat Atlanta 157-115 and secured sixth in the East, while Miami (8th) tuned up for a trip to the East's play-in tournament by beating Toronto 118-103.

The Heat will go to Philadephia (7th) on Wednesday with the winner getting the seventh seed and the loser to face either Atlanta or Chicago to determine the No.8 - who will take on Boston in the playoffs.

The Sixers beat Brooklyn 107-86 and expect to have rested superstar Joel Embiid (knee) back in for the post-season.

Golden State ended up 10th in the West after beating Utah 123-116 and will face ninth-seed Sacramento in the play-in.

- with AAP