The fast bowler was one of many England players to struggle on the opening day of the three-day game in Dunedin as the tourists were dismissed for 131 before the Selection XI established a 46-run lead by reaching 177 for four.
But Harmison, wayward at first, claimed 4-41 in a 10-over spell to return figures of 5-100 as the hosts were dismissed for 271.
By the close of the second day England had established a 15-run lead with Andrew Strauss’ unbeaten 55 guiding them to 155 for three.
“It’s got marginally better the more and more I’ve bowled,” said Harmison.
Having arrived later than the other specialists to attend the birth of his fourth child, Charlie, Harmison believes he is starting to catch up with his match fitness.
“At this moment in time I’m still trying to find my feet and find the right length on these wickets because I’ve only been in the country eight days,” he said.
“I wouldn’t have swapped coming here late for the world but I am the sort of bowler that does take time to being used to being in a different country and I feel I’ve acclimatised better than I thought I would which is a bonus.
“There were some quick balls, there were some good balls, but there some bad balls in there as well.
“But the more I’ve gone on during this trip, there have been more good balls in an over than bad balls, so I’ve got to be relatively happy.”
Harmison admitted that with Chris Tremlett getting treatment for a side injury that has cut short his tour, and Matthew Hoggard also off the field for a short time being sick, there was a question at one stage of who else would be available to take the new ball.
“We were scratching our heads at one point to look at who else was going to have a bowl,” he added.
“Hoggy was sick, Tremlett was off injured and we weren’t sure who was going to bowl with the new ball, but I enjoyed it and I feel like I’m getting better and to get five wickets is great.”