On 30 June 1975, Queensland ALP Senator Bertie Milliner died suddenly. The Labor Party nominated Colston to fill the casual vacancy in the Senate. The Constitution provides that a Senate casual vacancy is filled by a person chosen by the relevant state parliament. Although not a constitutional requirement until 1977, it was long a convention for the state parliament to choose a person nominated by the departing Senator's political party. However, the Premier of Queensland, Joh Bjelke-Petersen, claimed that Colston was a "dangerous socialist" and refused to appoint him. Officially, though, Bjelke-Petersen expressed doubts over Colston's integrity and instead appointed Albert Field, a member of the Labor Party who was staunchly opposed to the policies of the Gough Whitlam Labor government.