The letter should be in the file that Archways holds.
Also noticing these indexed :
From: Robert Field, Auckland To: [Willoughby Shortland, Esquire] Colonial Secretary Date: 23 March 1841
From: Robert Field, Auckland To: James Coates, Esquire Date: 28 June 1841
From: George Cooper, Treasury, Auckland To: [Willoughby Shortland, Esquire] Colonial Secretary Date: 13 September 1841
And the option to ORDER details brings up the file numbers, so you should be able to get access to the contents of the files. I think the ones above are at the Wellington Offices, but as I mentioned earlier, I am not familiar with the NZ admin systems, and I am confident that the NZ Rchatters will know the NZ system far better than me. I am very NSW Centric.
Here's the link to the RChat NZ resources Board
http://www.rootschat.com/forum/index.php?board=299 and a friendly welcome from their Moderator is at this link
http://www.rootschat.com/forum/index.php/topic,503066.0.htmlCheers, JM