A by-election was held for the South Australian House of Assembly seat of Custance on 23 June 1990. This was triggered by the resignation of former state Liberal leader and MHA John Olsen.
Results.
The Liberals retained the seat after preferences.
! scope="col" colspan="2" style="width: 130px"|Party
! scope="col" style="width: 17em"|Candidate
! scope="col" style="width: 5em"|Votes
! scope="col" style="width: 3.5em"|%
! scope="col" style="width: 3.5em"| ;templatestyles src="Template:Tooltip/styles.css" / ;±%
! colspan="3" style="text-align:right;" | Total formal votes
! colspan="3" style="text-align:right;" | Informal votes
! colspan="3" style="text-align: right; margin-right: 0.5em" scope=row | Turnout
! colspan="6" style="text-align:left;" |Two-party-preferred result
References.
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