I can find no reference so far to the Canterbury Infantry Regiment, however of the period of 7th of July 1917 the battle for Messines Ridge commenced,this attack was successful however it was retaken by the Germans in April of the following year. The South African Brigade took it back for the final time in October of 1918.
The battle for Ypres/Passchendaele took place from the end of July to early November so it is likely that it was at Messines ridge were the gassing took place.
I have not manged to find any reference so far to the 7th General Hospital you quoted but there were a number of British CCS (Casualty Clearing Stations) at St Omer and I have found a number of references to them dealing with gas victims.
Have a look here this will give you some info on the Messine Ridge Battle.
http://www.nzhistory.net.nz/war/new-zealanders-in-belgium/battle-for-messinesI suspect that the Rifle brigade would have been two small a unit to get a specifice mention in most books and would have been included as part of the Canterbury Brigade. My only reference which covers this area and timeline is "The ANZAC experience" by Christopher Pugsley and ex NZ Army officer who is currently a senior lecturer in military history at Sandhurst a very readable and un biased account of NZ service during WW1. It should be readily available from your local library.
My main areas of interest is Gallipoli.
Hope this helps
Alan NZ
Ps I see from posts whilst I have been researching this that some areas have already been covered.
Alan