Afterwards, Rees said: "Donington Park is definitely one of the sketchiest tracks in the wet that I've ridden. I'm starting to get comfortable in the wet around here and look forward to tomorrow's races, whether it's wet or dry."
With weather conditions continuing to get worse throughout the morning, the organisers decided to postpone race one to Sunday. The starting grid for race one was determined by the combined free practice times, meaning Rees would start from 12th position on the grid.
The opening race finally got underway on a drying track on Sunday, Danny Kent grabbing the advantage as the pack stormed into Redgate off the start. Rees held on to his starting position of 12th before making a mistake on lap one, dropping him to 27th position.
Approaching mid-race distance, with improving track conditions, Rees started making progress, moving up to 24th. As conditions continued to improve, he was able to lap faster than the riders in front of him and move up to 20th.
Going into the final lap, Rees had moved up one more position to 19th, which he held hold to the finish. He finished 3.5s off a points-scoring position. Billy McConnell was the race winner, after making a last-lap pass to beat long-time race leader Lewis Rollo. Another Kiwi, Shane Richardson, riding the second Astro JJR Racing BMW S1000rr finished the 10-lap race in 13th.
The second race of the weekend was held later in the afternoon on a dry track, as Rees started from 22nd position on the grid. Race one winner McConnell made the best start as he led off the line and Rees held onto his starting position as all the riders jostled for position going into Redgate.
As the riders approached the end of the opening lap, Rees was looking comfortable pushing hard to make up positions but, going into the Melbourne Loop for the first time, he crashed, ending his race early.
Richardson finished the race in 17th position, 33.868s off the race winner Neave.
After five rounds of the championship, Rees is now in eighth position on 71 points, 11 points behind seventh-placed Fraser Rogers.
The final round of the British Superstock 1000 Championship will take place at Brands Hatch over the weekend of October 17-18.