The rivals clashed in the debate on whether the UK can leave the EU, no matter what, by the October 31 deadline.
Asked for a guarantee he would do this, Boris Johnson described the deadline as "eminently feasible".
Michael Gove and Jeremy Hunt said extra time might be needed.
Mr Hunt, directing his comments at Mr Johnson, said that if the only way to leave was without a deal then he would do that, adding: "What would you say to a sheep farmer in Shropshire that I met whose business would be destroyed by 40 per cent tariffs? He would say, you got your dream of getting into No 10, but what about my dream to have a family business?”
Mr Dunne, who has pledged his support for Mr Hunt to become the next Prime Minister, said it was an interesting debate but wasn't handled in a way that allowed the candidates really to make their points very cleanly.
He said: "There was a lot of talking over each other.
"I thought personally that Jeremy Hunt did a very good job. I think he was very clear and got his message across, and of course he was the only one that mentioned Shropshire in the debate."
He said it was necessary to be very clear that the UK is determined to leave the EU with a deal if possible, but without if not.
However, he said he would be in favour of a short extension to the Brexit date if it looked as though a deal was on the table.
Mr Dunne said: "If we are through negotiating with the leaders of the EU in a position where a deal is in sight, when we get to the October council on the 16th, but for technical reasons it won't be capable of being ratified by all the EU states by October 31, then it would be bizarre not to have a short extension to allow that to happen."
He says he doesn't believe Mr Johnson, who has led the race, would make the best Prime Minister.
MPs were voting in the third round of the leadership contest this afternoon, with the results announced later today.
The five contenders are Sajid Javid, Mr Johnson, Mr Gove, Mr Hunt and Rory Stewart.
Former Brexit Secretary Dominic Raab was earlier eliminated in the second round of voting.