Aided by some outstanding centre play from Tupou, Johnstone scored a hat-trick in 10 first-half minutes during Trinity’s 35-18 win at Leeds Rhinos.
This took his tally for the season to six in four Betfred Super League games and though Chester believes the England man has room to improve he reckons there are few better finishers in the competition.
“His try scoring record is really good,” said the coach. “We have had a chat about the defensive side of his game, but he is a class winger.
“He scored some really good tries and carried the ball really well for us.”
Only Man of Steel Ben Barba scored more Super League tries than Johnstone last year when the German-born 23-year-old went on to make his England debut, but Chester feels he is still learning.
“He knows there are things in his game he needs to work on,” he added.
“He is not the complete player yet, but he is a player that excites me every time I see him wear a Wakefield Trinity shirt.”
Tupou has become a key figure for Trinity since being recruited in 2015 to help in their battle against relegation
“He’s got some really good ball,” Chester observed. “He set some tries up and I was really pleased for Bill. They are two really special players, both of them.”
Chester, though, will not be allowing his in-form pairing to pat themselves on the back.
“We were a little bit indecisive with some of our reads on that left edge,” he pointed out.
“There is plenty to work on.”
The coach was keen to stress the team effort involved in the win at Headingley
He said: “Collectively I was pleased for everybody involved. It was a really good week and we felt we got everything right during the week. That was evident in the game.”
The one concern for Wakefield was a knee injury to Matty Ashurst, who was another member of last year’s Dream Team. He was due to undergo a scan, but Chester admitted: “It doesn’t look good.”