The ultimate Warrior marked his milestone with a win, but a sternum injury soured Simon Mannering's 300th appearance in the Warriors' 20-16 NRL victory over Canberra tonight.

A penalty goal to halfback Shaun Johnson broke a 16-16 deadlock inside the final quarter and a second two-pointer with 11 minutes remaining gave the Warriors enough breathing space to fend off a fast-finishing green machine in front of a sell-out crowd of 24,595 fans at Mt Smart Stadium.

The Warriors' place in the top eight was already secured heading into the match, but they will need to improve drastically in week one of the playoffs after finishing the regular season with a disappointing and disjointed display.

"Given the occasion, whether that possibly had something to do with it I'm not 100 per cent sure but I didn't think we played our best footy," said coach Stephen Kearney.

"Although we didn't play so well we managed to find a way to get the job done tonight and it wasn't real pretty at times but sometimes that's what you've got to do."

The Warriors managed first half tries to left centre Solomone Kata and right wing David Fusitu'a but a four-pointer to Raiders forward Josh Papalii and two goals to halfback Sam Williams squared the scores at 8-8 at halftime.

The Warriors extended their lead with a try to Peta Hiku soon after the resumption and a Shaun Johnson penalty goal pushed them out to 16-8 in the 48th minute, but a try to Raiders back rower Elliot Whitehead and another Williams penalty goal made it 16-16 near the end of the third quarter.

Johnson banged over two goals to edge the Warriors clear but the Raiders kept coming and were unlucky after five-eighth Blake Austin knocked on in-goal and an obstruction ruling going went against them after Williams looked to have scored.

Every time the Warriors looked to have done enough the never-say-die Raiders conjured up a play that threatened to spoil the party and relief was the overriding emotion for the home side once fulltime blew.

The night was all about milestone man Mannering but it remains unclear whether retiring back rower will be fit to take part in the Warriors first finals campaign in seven years after he exited play wincing in pain less than two minutes into the second half.

Mannering's achievement makes him just the 34th player to reach the triple-century mark in the competition's 100-year history and only the 16th to do so for a single club.

The 32-year-old's health status will dominate storylines over the next week and the side will sweat on his availability while welcoming the return of first choice five-eighth Blake Green from a calf injury.

"He's pretty sore," said Kearney. "We'll wheel him out whenever we play next week and keep him out of contact this week that's for sure."

The narrow win moves the Warriors up into fifth spot on the premiership ladder on 32 points and leaves them in contention to host a home playoff game and extend Mannering mania in Auckland for another week.

Whether they remain there will hinge on other results throughout the weekend and their finals fate won't be fully determined until after the final match of the round between Brisbane and Manly on Sunday evening.

It remains to be seen whether the tense nature of the victory will benefit the side in the pressure cooker environment of finals football but if they wanted a reminder of what's required to get there - they certainly got it.

Warriors 20 (Solomone Kata, David Fusitu'a, Peta Hiku tries; Shaun Johnson 1/3 cons, 3/3 pens)
Raiders 16 (Josh Papalii, Elliot Whitehead tries; Sam Williams 2/2 cons, 2/2 pens)