Teenage striker Nathan Burns scored the easiest hat-trick of his life to book Adelaide United a major semi-final berth with minor premier Melbourne Victory.

The 18-year-old put away three simple tap-ins in a 3-1 defeat of a woeful Central Coast Mariners in Gosford.

The three points secured the crucial second spot on the table for United, enabling it two lives in the play-offs and the chance to host the grand final on February 18.

The major semi against Melbourne will be played over home and away legs, the first at Hindmarsh Stadium, Adelaide on Sunday night.

Newcastle Jets (3rd, 30pts) and Sydney FC (4th, 29pts) will fight out the sudden death minor semi-final, the first leg to be played at Aussie Stadium on Friday night.

The last game of the regular season took a long time to come alive.

Temperatures well above 30 degrees at Central Coast Stadium seemingly sapped the energy and spirit of players and fans alike.

But Adelaide did more than enough, considering it only needed a draw to secure second spot.

A late strike from Tom Pondeljak was merely a consolation for the hosts, but it could have been much more than that if teammate Adam Kwasnik had slotted his penalty chance just after the half.

The first Adelaide goal came in the 10th minute when Dutchman Bobby Petta was released down the right.

He rolled the ball across the face of goal to gift teenager Burns the simplest of finishes and his fourth goal of the season.

United went into the break leading 1-0 and immediately upon the resumption the Reds' Mathew Kemp handballed inside the box. Kwasnik hit the post with his penalty attempt.

Adelaide made the most of its luck, Burns on the spot to extend the visitors' lead in the 49th and 66th minutes before Pondeljak pulled one back from a Daniel Beltrame mistake late on.

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