Brock had worked in Port Pirie's lead smelter, which was eventually acquired by Nyrstar, since arriving in the town in 1976. He was first elected to the Port Pirie Regional 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.
Political career
2009 by-election
Brock had 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 council mayor. Independent Senator Nick Xenophon also campaigned for Brock. On 23.6 percent of the primary vote and 51.7 percent of the two-candidate-preferred vote, Brock's election depended 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, Greens, 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, with Brock close behind Labor. By 21 January 2009, both the ABC's Antony Green and the state electoral office were indicating a 2-point 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 picked up enough National and Green preferences to overtake the Labor candidate for second place by 30 votes. He then picked up enough Labor preferences to take the seat off the Liberals on a two-candidate-preferred vote of 51.7 percent, 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. Labor won from the Liberals the two-party-preferred vote on 50.1 percent.
2014 election
Brock increased his primary vote to 45.2 percent and two-candidate vote to 58.8 percent at the 2014 election. The election resulted in a hung parliament with 23 Labor seats, 22 Liberal seats, and two independents. The balance of power was held by crossbench independents Brock and Bob Such. Such did not indicate who he would support in a minority government before he was diagnosed and hospitalised with a brain tumour and took medical leave one week after the election. University of Adelaide Professor and Political Commentator Clem McIntyre said Such's situation virtually guaranteed Brock would side with Labor. With 24 seats required to govern, Brock backed Labor. McIntyre said:
If Geoff Brock had gone with the Liberals, then the Parliament would have effectively been tied 23 to 23, so once Bob Such became ill and stepped away then Geoff Brock, I think had no choice but to side with Labor.
Brock accepted the cabinet positions of Minister for Regional Development and Minister for State and Government Local Relations. Brock agreed to support the Labor government on confidence and supply while retaining the right to otherwise vote on conscience. Martin Hamilton-Smith resigned from the Liberals and joined the Labor cabinet two months after the election. Labor achieved majority government when Nat Cook won the 2014 Fisher by-election which was triggered by the death of Such. Despite this, the Jay Weatherill Labor government kept Brock and Hamilton-Smith in cabinet, giving the government a 26 to 21 parliamentary majority. In the Weatherill Ministry, between 2014 and 2018 Brock has served as the minister with responsibilities for regional development and for local government.
