Rangitata MP Andrew Falloon said it was a sensible change and made more sense to have all of the Ashburton district in one electorate.
Now, part of the district - Rakaia, Rokeby, Somerton, Chertsey, Pendarves, Dorie - is in Selwyn.
The commission said that the Rangitata electorate was 1.4 per cent below population quota, and projected to decline, and population of 3300 had been added from Selwyn.
Selwyn is 16.5 per cent over quota so must lose population, including the area south of the Rakaia River.
Mr Falloon said he relished the opportunity to represent communities like Rakaia, Chertsey, Lauriston, Pendarves and Dorie.
Throughout this parliamentary term he and retiring Selwyn MP Amy Adams had operated a "no-wrong-door'' approach to anyone on the Ashburton side of the Rakaia who had been in the Selwyn electorate, giving people the choice of coming to his office.
Mr Falloon said he thought he had put forward a compelling case to the commission.
