In 1980, he was elected to the Australian Senate as a Democrats senator for Victoria. He was, however, re-elected in the 1984 election, when seven places were up for election due to the expansion of the Parliament. In 1986, he left the Democrats, claiming that the party had moved too far to the left.
In 1987, he registered the Unite Australia Party, amalgamating two other minor parties, the Advance Australia Party and the remnants of the Australia Party.
He was joined in the Senate by South Australian Democrat David Vigor. The new party contested the 1987 election but received under 1% in all states contested.
Siddons and Vigor were both defeated.
He was a businessman and the executive chairman of Siddons Industries Limited. before entering politics. He was defeated at the 1983 election, when he was required to take second place on the Democrats ticket in deference to party leader Don Chipp.