As it says in the "Searching the Index ... Limitations" section about the Colonial Secretary's Letters "The index is not a complete list of all material within the series NRS 905". Perhaps an email to the NSW Archives might help? Looks like you need Reel 2934 Item (5/2400) .
https://mhnsw.au/articles/922/
The reel you mention is just an Index to the register of letters received, ie it gives a surname or subject and then the reference number, nothing more. You then need to go to the actual register of letters received to get more details, which I've already done - 44/2540 was dated 28 Mar 1844 from the Royal Engineers, ie
Appl[ication]n from Lt Hamilton RE Norfolk Island for use of Govern[men]t HouseI've been through the Index for the letter M, Jan-Jun 1844, and there's nothing at all for Mackley or anything similar.
I also looked up 44/2546 in case it was bad handwriting, but this relates to John McCurdy of Penrith regarding his wife Sophia, and is dated 26 Mar 1844.
I think you could be right though and that an email to the archives may be needed.
Thanks also to Christine for the Cecil Hills info - I'd done a little bit of searching but had planned to come back to it, so hopefully the database will be up and running again by then
