Smith was born on a South Canterbury farm near Methven. His father was a shearer and later owned his own farm. Smith began his career working on the family farm and purchased a half-share in his wife's family's Marlborough vineyard in 1994. Later, they began making and selling their own wine.^[2] Their winery, Fairhall Downes Estate, was sold in 2015.^[3]

Smith was on the board of the New Zealand Winegrowers Association, including for six years as chair from 2006 until 2012.^[2] Smith's time as chair coincided with a downturn in the New Zealand wine industry.^[4] Smith opposed measures proposed to support the industry, such as its consolidation into cooperative models similar to Fonterra and Zespri, stating that more formal cooperation would not improve strained relationships between grape growers and winemakers.^[5]

It was reported during Smith's tenure as chair of the Winegrowers Association that he was considering running for Parliament in the Kaikōura electorate.^[6] In 2013, Smith challenged incumbent MP Colin King for National's nomination to contest the electorate. He won the nomination and, in the 2014 general election, was elected with a majority of 12,570 votes.^[7] Smith has held the Kaikōura seat ever since, returning to Parliament in each of the 2017,^[8] 2020,^[9] and 2023^[10][11] elections.

In his first term, the last of the Fifth National Government, Smith was the deputy chair of the primary production committee (from 2014 to 2017) and the social services committee (in 2017). From 2017 to 2023, the National Party was the official opposition. Smith held party spokesperson roles in the civil defence, Earthquake Commission, immigration, viticulture, climate change, and energy and resources portfolios.^[12]

Two members bills promoted by Smith were debated in Parliament in 2020 and 2023. Smith's Insurance (Prompt Settlement of Claims for Uninhabitable Residential Property) Bill proposed imposing a six-month time limit for insurers to make decisions and notify claimants on claims relating to earthquake damaged buildings.^[13] The bill, first lodged in 2018, was progressed to select committee consideration in July 2020 but Smith later withdrew it after the committee found the government was already undertaking work to address the issue.^[14] In 2023, the Sale and Supply of Alcohol (Cellar Door Tasting) Amendment Bill was introduced. This bill proposed regulatory reforms related to cellar door operations at wineries and was passed at its first reading in a 105–9 vote.^[15]

In the 54th New Zealand Parliament, Smith is the chair of the finance and expenditure committee.^[16]

Smith voted in favour of decriminalising abortion and legalising euthanasia.^[17][18]

Smith is married to Julie, a former occupational therapist. They have three children.^[2]