Australia marriage index shows a Martha Kerr married in 1930 Perth, Western Australia - no husband details are shown BUT
also shows an entry for a Joseph M. Gillespie - no wife details, not even a year, in Perth, W.A.
HOWEVER
the registration number for both entries is 412.
Think this must be your couple.
You state that youngest child Donald shown as Kerr in 1930 but the name of the child is Thomas G. Kerr - however, Australia Death Index shows a Joseph Donald Gillespie b.1932 d.1975 in Perth, Western Australia - parents shown as Joseph M./Martha.
A lot of the entries for Perth are very 'bitty' with lots of blank boxes. Cannot see death entries for Joseph M. or Martha but suspect the lists are not complete.
There are 2 birth entries on SP for 1925 births in Campbeltown - a plain John Gillespie and also a John McAulay Gillespie. If I'm reading shipping lists correctly looks like Joseph and indeed whole family went to Australia on 11/2/1926 when Martha had already had John. He obviously left her pregnant as Thomas Gillespie Kerr's birth is registered in 1926 (probably latter part of year).
Annette