Geoffrey Graeme "Geoff" Brock is a South Australian politician, representing the seat of Frome in the South Australian House of Assembly as an Independent, after a shock win at the 2009 Frome state by-election, defeating the Liberal candidate Terry Boylan. He had a high local profile prior to the election, having served for almost six years as the Mayor of the Port Pirie Regional Council. Independent Senator Nick Xenophon also campaigned for Brock.
On 23.6 percent of the primary vote and 51.7 percent of the two-candidate vote, Brock's election was dependent on preferences from Labor, Nationals SA, and the SA Greens, the former two having placed him second on their how to vote card. His own how to vote card saw him preference the Nationals, Labor, Liberal, Green, and One Nation, in that order. The by-election was closely contested, with the result being uncertain for over a week. Initial reports suggested a slight swing to the Liberal candidate Terry Boylan on the two-party preferred count against Labor, but with Independent candidate Geoff Brock not far behind Labor. By 21 January 2009, both the ABC's Antony Green and the State Electoral Office were indicating a two percent swing against the Liberals toward Labor on 51.4 percent, but not enough to lose the seat. Liberal leader Martin Hamilton-Smith claimed victory on behalf of the party.
However, the result hinged on the performance of Brock against Labor in the competition for second place. Brock won the primary vote in the Port Pirie area and received enough preferences, ending up ahead of the Labor candidate by 30 votes. He then received enough preference votes from Labor and other candidates to achieve a two-candidate preferred vote of 51.7 percent (a majority of 665 votes) against the Liberal candidate, despite a slight improvement in the Liberal vote since the previous count.
Brock increased his primary vote to 37.7 percent and two-candidate vote to 57.5 percent at the 2010 election.
Brock had worked in Port Pirie's Nyrstar smelter since arriving in the town in 1976. He was first elected to the Council (at that time a City Council) in 1989, and served on numerous community committees before being elected Mayor in May 2003, defeating sitting mayor Ken Madigan by 3,297 votes to 2,173. He retired from Nyrstar in September 2007, and he and his second wife Lyn have 12 grandchildren between them.