Hargest returned to New Zealand in May 1919 with his wife, Marie Henrietta Wilkie. The couple had been married since 1917, the ceremony taking place in England where Marie was serving as a nurse in the New Zealand military hospital at Brockenhurst. Hargest returned to farming, buying land near Invercargill. He retained an interest in the military and resumed his career with the Territorials in which he commanded firstly a regiment and then an infantry brigade. An interest in local affairs soon developed and Hargest became involved with several local authorities including the Southland Education Board. In the , he contested the electorate standing for the Reform Party and came very close to beating Sir Joseph Ward. The former Prime Minister had a majority of 159 votes, which represented a 1.5% margin. The death of Sir Joseph triggered the August , which was contested by Hargest and Ward's second son, Vincent Ward. Hargest was beaten in by Ward Jr., who had a majority of 571 votes (5.82%), and Hargest had thus been beaten by both father and son. Ward Jr. retired at the end of the term, and this allowed Hargest to enter the New Zealand Parliament in the 1931 general election on his third attempt, becoming the MP for the Invercargill electorate. In parliament, Hargest was an advocate for the interests of Southland but was also interested in defence and educational matters. He held this electorate until 1935 before successfully switching to the Awarua electorate for the 1935 election. Initially an Independent Reform MP, he was a supporter of the coalition between the United Party and the Reform Party. When the coalition combined to become the National Party, Hargest formally joined the new party and was "possibly the Reform MP most committed from the first to the formation of the National Party". In its early period of the National Party, there was a lengthy discussion about its leadership, as the previous leaders of the constituent parties were not acceptable to the other. At the time, many South Island MPs would meet at the home of Christchurch property developer Henry G. Livingstone after arriving on Saturday mornings on the overnight ferry from Wellington; Hargest, Adam Hamilton, and Sidney Holland belonged to that group. At the first official meeting of the party's Dominion Council in October 1936 in Wellington, Hargest joined the executive committee. Following that meeting, the leadership question resulted in a contest between Hamilton and Charles Wilkinson. Former Reform Party leader Gordon Coates and other MPs sided with Hamilton and issued a press statement that bordered on blackmail, and Hargest wrote to Coates, rebuking him for his stance and pleading for unity, as the new party was still fragile.