The Rhodes Brothers lived in Christchurch, so an overseer at The Levels had considerable freedom to get around. I suspect that that Sidebottom had investigated the area west of Cave much earlier. He was active and ambitious man, so it is natural that he would have explored further inland looking for land that was not controlled by the existing runholders. The Mackenzie Pass is visible from several places closer the coast, so if he was curious, he would have wondered where it led.
While based at Cave, he would be well placed to travel up the Tengawai River towards the Waratah Valley. When he got to the Mackenzie Pass he would climbed to the summit and seen the wide plain on the other side. He might even have ridden down and explored the flat land he had discovered.
Being the first to discover this large area of flat land, he would begin thinking about how he could get control of it. Getting a pastoral license was relatively easy. The tricky part was getting sufficient sheep to stock such a big area of land.
Small numbers of sheep had gone missing from The Levels before the Mackenzie incident. Sidebottom had probably paid someone to drive them up through the Mackenzie Pass and release them. (This is probably why his letter suggested that Mackenzie had stolen sheep on previous occasions). He may have already worked out the route through Cannington and Mawaro, and the good places to cross the rivers.
Taking a thousand Merino’s may have been the next big step towards stocking the run he planned to establish. He had probably done a deal or formed a partnership with the person who had paid Mackenzie to drive the sheep. He might have called Seventeen back to Cave, so that they were easy to take.
Once the sheep were stolen, Sidebottom moved so slowly to give them a chance to get away. Once they were up through the Mackenzie Pass and scattered across the plain, it would be impossible to find them. He could then go and apply for a pastoral license, knowing that he already had sufficiently sheep to stock the area.