Marshall has been dubbed "Benchy Marshall" in some circles because of his lack of game time with the Auckland Blues, despite his high-profile and much hyped cross code switch to rugby union.

The rugby league premiership-winning playmaker will play his first Super Rugby game in Australia when the Blues clash against the ACT Brumbies at Canberra Stadium on Friday night.

Some have already called for the Blues to abort their Marshall experiment, lambasting him for a lacklustre 30-minute cameo from the bench last weekend as he attempts to find his feet as a fullback or flyhalf.

But former flyhalf Mehrtens, who played 70 Tests for the All Blacks and in four Super Rugby titles with the Crusaders, rated Marshall's transition a "seven or eight" out of ten.

"He hasn't had many opportunities. I think [the critics] need to cut him a bit of slack," Mehrtens told Rugby HQ.

"He's learning a new game, there are a lot of skills that cross over from league but you've got to be familiar with the game.

"[The Blues] played him at No10 in the pre-season and i thought that was a bit unfair. You don't want a guy like that who is a natural ball player and skilled guy to be tied down by organisation.

"Give him some freedom, put him at No15. He wants to run angles and once [the Blues] get used to running off him it will make a massive difference.

"I remember playing against [Spencer] and he was a genius with the ball in hand. But he wasn't effective until the other guys started to learn to run off him and react to his play.

"[Marshall] just needs more game time. I think stick with him, give him more and more time on the field. He may not be a starting XV guy but give him time on the field and have a bit of patience with him."

Brumbies captain Ben Mowen has backed Marshall to make a successful transition as the ACT prepares to take on the Blues.

Mowen said the harsh assessment was "a bit rough after seven games" and the ACT side is preparing to tackle some unpredictable Marshall brilliance.

Marshall has never lost a game at Canberra Stadium, winning all six NRL matches against the Canberra Raiders in his 201-game career with the Tigers.

"He's experienced and highly skilful. I think people are expecting a lot after seven games of rugby from a guy who has to come on and control a game," Mowen said.

"In terms of [playmakers'] decision making, in terms of when to push themselves into the line ... to come straight across and be expected to adapt is a bit too much for him.

"I think he's a guy who can make it. He's already shown he's got some classy touches, but all those things take time."

The Blues are the most attacking side in Super Rugby and average the most points per game - 30.7 - of any team in competition.

Marshall's impact has been limited in his one starting appearance and three games from the Blues' bench.

He's scored one try in his 146 minutes of his debut rugby season.

But Canberra has been a happy hunting ground for Marshall and the Brumbies will be hoping a trip to the capital doesn't ignite his Super Rugby career.

Marshall blitzed the Raiders in a 2010 NRL semi-final in Canberra, turning on a clinic to lead the Tigers to a tight two-point victory.

The game was largely remembered for Green Machine skipper Terry Campese's first major knee injury and Jarrod Croker's missed penalty attempt to level the scores in the dying minutes.

Marshall returned as tormentor in 2012 to humiliate the Raiders again in a 49-12 rout, kicking seven goals, scoring a try and slotting a field goal.

The 29-year-old is a dangerous weapon in the Blues' back line, which boasts All Black talent in Ma'a Nonu and Charles Piutau.

"They've got individually brilliant players," said Brumbies director of rugby Laurie Fisher.

"They're a dangerous team that can play tight or they've got the capacity to beat you one on one."