Mark Cueto and Chris Ashton both claimed hat-tricks in Dunedin on Saturday as England ran in five tries in each half to fell the Mighty Oaks in comprehensive fashion.
Ben Youngs, Ben Foden, Manu Tuilagi and Tom Croft all joined in on the try-scoring action as England placed one foot into the World Cup quarter-finals. England's place in the last eight will be confirmed if Scotland beat Argentina on Sunday.
But Johnson's men would still need to beat Scotland next weekend to win the group and avoid a quarter-final against New Zealand.
"These games can be awkward," said Johnson.
"We have to go out and play, keep our shape and keep our discipline, which I thought we did generally.
"If we did that we thought the opportunities would come with the tries, which they did. I am pretty happy, it is job done.
"We go into the last week with three wins, which is where we wanted to be. It's a real big week and hopefully we will get the performance to win that one.
"(The World Cup) really starts in a way. That will be a scrap - hopefully tomorrow it will be an incredibly physical game!
"It's a mental thing this week, getting ourselves ready for the game. It's going to be a big one."
Cueto, who was starting his first match of the tournament after recovering from a back injury, had scored just one try in his previous 25 Test appearances.
But the Sale wing came roaring back into form to run in his second Test hat-trick, and the third by an England player at the World Cup. Ashton then completed the fourth.
"When you play on the wing your opportunities often come from other people's good work," said Johnson.
"There were some really crisp hands and he was the benefactor of that on the left wing early on.
"When he wasn't scoring it wasn't a big deal because he was playing well."
Johnson also reserved special praise for Tuilagi, who was too hot for the Romanian defence to handle and scored his fourth try in just five Tests.
"When we picked him everyone talked about brute force and lack of creativity but the guy can play rugby," said Johnson.
"Some of the tries he has scored have been about great lines of running and being a good rugby player.
"He works very hard and he tries to get better all the time. The improvements he has shown with his club and with us have been fantastic.
"He wants to be the best rugby player he can be and it is good to have him."
Johnson blasted England for their poor discipline in the victories over Argentina and Georgia but he was content today, even though his men still lost the penalty count 12-11.
"I was a lot happier with the discipline. The referee has to referee this game differently than he would other things," said Johnson.
England were warned by referee Romain Poite after Jonny Wilkinson twice changed the ball before attempting a conversion.
"Wilko thought one of the balls was slightly less than perfect so he wanted to swap it but the ref said we can't do that so we didn't and we got on with it," said Johnson.
"The detail of the ball when you are a second row is not that important, the detail of the ball when you are kicking it off the touchline is.
"If they feel they don't like a ball they will try and change it. If the ref says no, the ref says no, that's fine.
"I didn't know about it until after the game but it is not an unusual occurrence."
There was an issue at the 2007 World Cup when Wilkinson was twice presented with a practice ball, rather than an official match ball.
A World Cup spokesman said: "England were warned at half-time as they were attempting to use a different ball for kicks at goal.
"They are supposed to use the same ball for conversions with which they have scored the try. It happened twice and they were told to desist, which they did."
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