Well I have been looking at the electoral rolls.
Searching for Elsie Porter I found two. The one living in Christchurch had a husband Charles and I found the marriage in 1915 of Elsie Hines to Charles Porter. The other one lived up North and her husband was William.
Next I searched for Donald Kerr Porter and found one living alone and in all the rest, the wife was Edith Hinemoa Porter.
So what you have does not match up.
How did you come by the info that Patrick Strathnairn Valentine Porter was the son of Elsie & Donald Kerr Porter?