People in the traditionally safe Liberal seat in the Adelaide hills voted for Nick Xenophon Team member Rebekha Sharkie in 2016 on the back of support for the former senator.

But Ms Downer said many people had told her they deeply regretted their decision and were now looking for representation that delivered.

"I think that people sometimes are curious about new things and they want to give it a try but people have tried it and found it a very disappointing experience," she told ABC radio on Monday.

"They want someone who delivers for them, who listens to them, and is part of a party of government that can be a strong advocate in that party."

Ms Downer, a former diplomat and daughter of former foreign minister Alexander Downer, said the mood in the electorate, which was held by her father for more than 20 years, was entirely different than two years ago.

The Liberal candidate said people had seen the limitations of being represented by a minor party.

Ms Sharkie, who was forced to quit this month after being caught up in the dual-citizenship saga, said high energy prices and the rebates given to those on lower incomes, would be a critical issue in the campaign.

"If we are genuinely worried about the cost of living and power prices why would we want to take the energy supplement away from our most vulnerable residents," Ms Sharkie said as she faced off with Ms Downer for the first time.

The vote in Mayo will take place on July 28 along with by-elections in Tasmania, Queensland and Western Australia.

Australian Associated Press