Hi Michelle
You don't give your grandfather's name or any timeframe so I couldn't do a lookup for you on NAAIRS, a searchable archive index for South Africa. However, I did a quick search for you on the surname DUNSTAN and came back with 300 hits!!!
I did find this which may be of interest to you:
DEPOT TAB SOURCE MHG TYPE LEER VOLUME_NO 0 SYSTEM 01 REFERENCE 95455 PART 1 DESCRIPTION DUNSTAN, ALFRED PEARCE. STARTING 19360000 ENDING 19360000 REMARKS SURVIVING SPOUSE ELIZABETH MORHAM DUNSTAN (BORN MURIE) PREDECEASED SPOUSES (1) MARGARET DUNSTAN (BORN BREKIE BANK) (2) ELIZABETH DUNSTAN (BORN MILLS).
If you want to look up any more DUNSTANs on NAIIRS the website is:
http://www.national.archsrch.gov.zaOnce you have found them you can either join the Rootsweb mailing list for Immigration to South Africa or hire a researcher to look up the documents that you need.
Regards
Kathryn
Abu Dhabi