THEY'D been the best side all season, but the minor premiership will be little consolation for the Canberra Raiders under-20s after they succumbed 36-20 to Penrith in Saturday's preliminary final.

A crucial late defensive lapse proved the killer blow for Canberra after it had launched a spirited second-half comeback.

Down 24-6, the Raiders piled on three unanswered tries to close to 24-20 with 12 minutes left.

But Canberra were made to pay for ball-watching moments later, five-eighth Blake Hewitt scoring a solo last-tackle try to break the Raiders' hearts.

''That try really hurt us,'' Raiders coach Mick Mantelli said. ''I knew we were still in the hunt [at half-time] we just needed to get across the stripe to get a bit of confidence. Before the game we spoke about little lapses, but we showed a lot of character, which we have all year.''

Searching for back-to-back grand-final appearances, the Raiders were made to pay for a poor first half-hour when they allowed a dominant Panthers to build an 18-0 lead.

With Holden Cup player of the year Bryce Cartwright almost unstoppable, only a lack of patience prevented Penrith from sealing the result by half-time.

Tries to Kieren Moss, Brendan Attwood and Mitchell Clark put Penrith in the driver's seat from the outset. But they were forced to withstand a Raiders revival before booking a grand-final date with New Zealand Warriors next week.

Despite being completely outplayed in the first half, particularly in the ruck, Canberra trailed just 18-6 at half-time after centre Andrew Heffernan scored just before the break.

And when Penrith centre Waqa Blake finished off a frantic last-play movement, the Panthers looked home ahead 24-6 with 25 minutes left.

But with back-rower Tevita Pangai (190 metres) providing the inspiration, the Raiders looked set to run over the top of Penrith, scoring three tries to bridge the gap to 24-20 after 68 minutes.

However, Penrith five-eighth Blake Hewitt exposed some lazy Raiders defence to score the match-winner late in the contest.

Panthers fullback Moss showed why he's so highly rated with a big performance, while prop Andy Saunders (152 metres) and wingers George Jennings and Dallin Watene-Zelezniak were also outstanding.

Returning from a hamstring injury, Pangai was Canberra's standout after running to the brink of exhaustion in the second half to lift his team.

AT A GLANCE

Holden Cup: PENRITH PANTHERS 36 (B Attwood 2, K Moss, M Clark, W Blake, D Hewitt tries; B Cartwright 6 goals) . CANBERRA RAIDERS 20 (J Reuben 2, A Heffernan, R Garard tries; M Cornish 2 goals) at Allianz Stadium.