Can place "James DONNELLY - Police Constable" at Masterton (Wairarapa) in March 1868, and "James DONNELLY, District Constable" April 1877- ("Manawatu Times") Palmerston North, from where I think he went on to Wanganui.
And while there were other police officers named DONNELLY referred to in the newspapers without a christian name, I believe the Sergeant DONNELLY involved with a Constable BUCHANAN at Wanganui (1879 and earlier), is James DONNELLY.
You'll need to read through these Court cases to get "the drift".
"Wanganui Herald"
- 18 January 1879 - R.M. Court
- 20 January 1879 - page 2 - R.M. Court
{There may be a couple of additional references in other papers too ? }
One of the above items confirms the separation of Sergeant DONNELLY and his wife - but she 'ain't named in any of them. Grrrr !!
"Wanganui Chronicle" - 1 February
1879 - Local and General
-
Police Changes ... reports
Sergeant DONNELLY is to be transferred to Wellington. ~ Lu