New Zealand were on 3-601 at stumps on Friday following a dominant day at the crease after starting off at 1-174.
Conway scored 153 before Nicholls and Ravindra compiled an unbeaten 256-run partnership to ensure a mammoth total.
Nicholls was 150 not out and Ravindra unbeaten on 165 at the close as the tourists resisted the temptation to declare late in the day.
The home team, bowled out for 125 in two sessions on the opening day on Thursday after winning the toss and choosing to bat, took only two wickets on a day of miserable toil as bat totally dominated ball.
Conway brought up his fifth Test century off 142 balls and then went on to hit 18 boundaries before being bowled by Blessing Muzarabani after Zimbabwe had taken the second new ball.
Before lunch, nightwatchman Jacob Duffy, one of three debutants in the Kiwi line-up, scored 32 before chipping to midwicket off the spin of Vincent Masekesa.
Duffy put on 73 with Conway for the second wicket while Nicholls and Conway added a further 110 before Conway's dismissal.
Nicholls, in the team in place of injured captain Tom Latham, took 166 balls to bring up his 10th ton in Tests while fellow left-hander Ravindra then upped the run-rate, needing only 104 balls for his third Test hundred.
New Zealand completed their nine-wicket win over Zimbabwe in last week's first Test inside three days and are expected to declare either overnight or early on Saturday, leaving Zimbabwe with a massive deficit to try and haul in if they are to avoid another heavy defeat.
