Her baptism on Family Search, with her sister Eleanor:

Name: Nesta Isabella Hart Powys
Event Type: Baptism
Event Date: 18 Oct 1899
Event Place: Cathcart, Cape of Good Hope, South Africa
Gender: Female
Father's Name: Robert Vernon Powys
Mother's Name: Wihlemina Powys
Digital Folder Number: 004467532 , Image Number: 00043

Her birth date of 16 May 1895 was included in the image.
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Her baptism with birth date was on the same image as that for Eleanor Mona Powys and provided by Ancestry, as well as Family Search.  Transcript by me for the joint entry:

>              Cape of Good Hope  >  Cathcart  >  Baptisms 1880-1969

“BAPTISMS solemnised in the Wesleyan-Methodist Chapel . . . .
“   in the County of  . . . . .  in the Year of 1899
“. . . .
“When Baptised:       October 18th. 1899
“No:                  660
“Parents’ Forenames:  Robert Vernon and Wihlemina
“Parents’ Surname:    Powys
“Abode:               Cathcarl  or, on other pages, Cathcart
Celebrant:            Geo H.P.Jacques

                      First child            Second child
                      ===========            ============
“Child’s Forename:    Nesta Isabella Hart    Eleanor Mona
“When born:           May 16th 1895          March 29th 1899
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