He was elected as the represented the Heretaunga and served as an undersecretary and Minister of Justice. He was convicted of untrue statements to investors and sentenced to house arrest and community service. Justice Dobson was of the opinion that Jeffries believed his statements but the law required accuracy not belief.
He represented the Heretaunga electorate from 1981 to 1990, when he was defeated by a National candidate in a swing against Labour.
He was undersecretary to the Minister of Transport in 1986 and also to the Minister of Works, and chairman of a parliamentary committee on road safety, in 1987. In April 1988 he was appointed chairman of the National Roads Board.
He was Minister of Justice from 1989 to 1990 in the Fourth Labour Government. He was Minister of Justice from 1989 to 1990 in the Fourth Labour Government.
On 24 February 2012 Jeffries was convicted, along with fellow former Justice Minister Sir Douglas Graham and two other men, of breaching the Securities Acting by making untrue statements to investors in his capacity as a director of Lombard Finance.
Justice Robert Dobson wrote, "I am satisfied that the accused genuinely believed in the accuracy and adequacy of the.. documents", but that the offences were ones of strict liability so there was no need for "any form of mental intent to distribute documents that were false or misleading". Jeffries was sentenced to 400 hours community service. The Court of Appeal dismissed his appeal against conviction and increased his sentence to eight months home detention and 250 hours community work, but the Supreme Court restored the original sentence.
Retired Court of Appeal judge Sir Edmund Thomas described his convictions as a "grievous miscarriage of justice", saying of the crucial piece of evidence that "you would never ever convict a dog on the basis of the schedule".