"The real test for us will be with the ball in hand and our timing. You can't help it, [timing] is just going to be off.
"I had three weeks off during the season and when I came back it just didn't feel right so it's going to take a game to get that back. It just sort of sucks that it's against Australia."
With head coach Stephen Kearney and assistant David Kidwell dividing their time between training sessions in Brisbane and Sydney, the New Zealand-based group have been put through their paces by Warriors trainer Andrew Souter.
"He's a guy that we all know pretty well and he's done a really good job. Just a lot of short and sharp sessions with a lot of repetitive speed work and using the ball while you're fatigued. We do a lot of work and then try and get out and try and do some skill."
Both Johnson and newly approved New Zealand prop Suaia Matagi attended the NZRL's unveiling on Friday of a new Kiwis jersey, which features a motif inspired by Maori and Pacific influences.
The new design struck a chord with Matagi, 26, who last week had his change of allegiance from Samoa to New Zealand approved by the Rugby League International Federation.
He was excited at the possibility of lining up against his old team in what could be one of his first outings for the Kiwis, when they meet Samoa in Whangarei for their second match.
"Just seeing it now for the first time, it would be a real honour and hopefully I can pull on the jersey against Samoa. That would mean a lot," Matagi said.
"This is a dream come true for me, just to be in the squad. To finally learn the IRLF have given the thumbs up, I'm really excited to get into camp."
Despite Samoa initially protesting against New Zealand's claim to the damaging front-rower, Matagi said he had made the switch to the Kiwis with their approval.
"They understood the opportunity that was there for me and I had a good talk with Samoa coach Matt Parish and he told me to go for it."
Should he line up against his former countrymen, Matagi will sing the national anthems of both countries.
"That's what I did at the World Cup when we played the Kiwis. I was singing the Kiwis anthem in my heart. I'm New Zealand born and proud to be Samoan too so I'll sing both."
The Kiwis open their Four Nations account against Australia at Brisbane's Suncorp Stadium on October 25.