The bellwether seat will increase in area by about one-third, to 41,000 square kilometres, the size of Switzerland.

Eden-Monaro is based on Queanbeyan and extends to the south coast and the Victorian border.

However, under the changes made by the Australian Electoral Commission, it will extend past the ACT to take in the Yass and Tumut shires.

Eden-Monaro loses Batemans Bay to Gilmore but gains 14,600 voters from the Hume electorate in the Yass valley and the Palerang local government area, and 10,500 electors from the Riverina electorate in the Tumut and Tumbarumba local government areas.

Redistributions in NSW were forced by the decrease in the number of the state's federal electorates, from 48 to 47 seats.

Eden-Monaro is considered a litmus test for the election outcome because, since 1972, the electorate has always been won by the party that forms government.

As boundary changes for the next election come into effect on Thursday, the electorate will become slightly safer for the sitting Liberal MP, Peter Hendy.

He was elected in 2013, defeating Labor's Mike Kelly when Tony Abbott swept to power, and is now a parliamentary secretary.

He will now hold the seat by an estimated margin of 2.6 per cent, according to the ABC's Antony Green, compared to 0.6 on the old boundaries.

Dr Hendy said despite the addition of rural constituents, he regarded the seat as marginal.

"I'm confident the government will retain Eden-Monaro at the election but I'm not complacent about it," he said.

"Geographically, the electorate has expanded enormously.

"Previously I would do a three-hour 20-minute drive south [from Queanbeyan] to Eden, the furthest major town in the electorate.

"Now I have a three-hour 20-minute drive west to Tumbarumba.

"Geographically Eden-Monaro will be equivalent to countries like Switzerland or Holland or Denmark and will be bigger than 65 countries."

Dr Kelly who held the seat from 2007 to 2013, and is the Labor candidate, said he was positive about the redistribution while admitting the winner would have a huge physical challenge to stay in touch with the far-flung electorate.

"This gives us the opportunity to create a regional strategic economic plan where we all work together," he said.

Dr Kelly said although he was the underdog, he might be helped by his hard work as an MP for voters in the Tumut and Tumbarumba shires when they were part of Eden-Monaro in 2007.

"That resulted in significant swings in all the town centres and shows that they are an open-minded community," he said.

"It's a massive physical challenge because of the diversity of the region."