In the second practice round he improved to 1m 40.558s but ended in 25th position, 3.345s off the fastest lap set by Chrissy Rouse.
Saturday was a busy day with qualifying in the morning and the first 14-lap race in the afternoon.
In qualifying, Rees set his fastest lap of 1m 39.025s on his eighth circulation, putting him in 17h position on the grid for race one.
Fellow Kiwi, Shane Richardson qualified 11th, with a lap time of 1m 38.386s.
Race one got off to a quick start with Rouse grabbing the hole-shot, ahead of Rollo and Kent, by the end of the opening lap Rees had managed to hold on to his starting position and was following TT race winner Dean Harrison. In lap three, Harrison crashed out promoting Rees to 16th.
By lap seven Rees was in a four-way battle for 15th with Luke Jones, Davey Todd and Luke Hopkins. Lewis Rollo would take the win by 4.8s and claim his first victory in the British Superstock 1000 Championship.
After a tough race Rees finished 17th just 0.222s behind 16th. With his first non score of the season Rees slipped from 3rd to 5th in the Championship, just three points behind third-placed Tim Neave. After a strong ride and finishing second, Rouse became the new championship leader.
In race two on Sunday afternoon, Rees started from 19th position.
At the end of lap three he made a move into 13th position just 0.7s behind fifth. By the race's midpoint Rees had dropped back to 14th position and was in a three-way battle for 13th with David Allingham and Luke Jones.
By the start of the penultimate lap Rees had passed Luke Jones for 11th, which he would hold to the finish. Shane Richardson finished in 13th position just 0.272s behind Rees.
"The toughest thing definitely had to be the technicality of the track," Rees said.
"It's very blind and undulating. This means that it's not a straightforward track to learn, you really need to take your time with it.
"The season had gotten off to a far better start than I think anyone expected, I was pretty disheartened by my qualifying and race one results. But I'm a fighter and always want to improve so I chose not to give up and took what I can from it."
After four rounds Rees is in sixth position in the National British Superstock 1000 Championship sitting on 71 points, 18 points behind third position.