A ReachTEL poll of 1031 voters shows Sharkie has a 58-42 lead over Downer on a two-party preferred basis.

The poll, taken on June 5 for the Australia Institute in the wake of a flying visit to the seat by Prime Malcolm Turnbull, shows Sharkie with a primary vote of 40.1 per cent, Downer on 34.4, Labor on 7.7 and the Greens on 10.7.

The poll mirrors another commissioned by Channel 7, also conducted by ReachTEL, from a sample of 831 local voters.

Sharkie won Mayo as a Nick Xenophon Team candidate in 2016 before resigning over dual-citizenship and is recontesting the by-election for the party, rebadged as Centre Alliance.

Downer is seeking to follow in the footsteps of her father, Alexander, who held Mayo from 1984 to 2008.

With the vast majority of Labor and Greens preferences likely to flow to Sharkie, Downer will have to substantially increase her primary vote to have any hope on July 28.

Sharkie won the seat two years ago with 55 per cent of the two-party preferred vote. Her primary vote was 34.9 per cent behind Liberal incumbent Jamie Briggs on 37.8 per cent, which indicates the strength of the preference flow to her.

Increasing the degree of difficulty for Downer is the fact that most voters in Mayo appear to have already made up their minds: the latest poll shows only 3.6 per cent of voters were undecided.

Telecommunications consultant and academic Professor Reg Coutts was named as Labor’s candidate yesterday – after the poll was taken.

He won preselection over Alice Dawkins, the daughter of former federal treasurer John Dawkins.

Coutts told InDaily yesterday that he was in the race to ensure the “Downer dynasty” didn’t return to Mayo, adding that Sharkie was doing a good job as the local MP.

The poll also asked Mayo voters about whether the tax rate for “large companies” should be increased, kept the same or decreased.

About a quarter of voters (25.4 per cent) favoured in increase, slightly less (24.8 per cent) wanted a decrease, while 44.9 per cent favoured no change.

– with AAP