Winston Peters cut a swathe through the wealthier parts of the Northland electorate in Saturday's byelection, securing the most votes in the National bastion of Kerikeri and matching the number cast for the ruling party in Wellsford.

While the figures relate to the less important constituency vote, rather than the crucial party vote, National Party strategists will be concerned by the extent of Peters' reach into areas where support for their party has traditionally been rock solid.

An index of deprivation produced for a profile of the electorate compiled by the Parliamentary Library shows that the better-off areas of Northland are in the southern part of the seat plus the hinterland around Kerikeri more to the north.

Any booth-by-booth analysis of Saturday's result is complicated by the high level of advance voting in the seat ahead of byelection day - up by more than 3600 votes on the 9478 cast by early voters at September's general election. Nearly half of the total votes in the byelection were cast in advance.

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National's 2014 candidate, Mike Sabin, secured 60 per cent of the votes cast in Wellsford at the general election - well above the 53 per cent he averaged across the whole electorate. National's share of the advance vote in Wellsford in the byelection was 47 per cent, just below the 48 per cent recorded by Peters. However, National's Mark Osborne narrowly won the most votes cast in the town's polling booth.

The New Zealand First leader won the advance vote in Kerikeri along with the votes cast in the town's three polling booths. National's vote in those booths fell from just over 1300 last September to 821 on Saturday. Part of that gap can be explained by advance voting. But part of it would also have been due to defections from National to New Zealand First.

Peters won 60 per cent of the advance vote in Dargaville and was close to that mark with respect to votes cast in polling booths. Last September, Sabin picked up 58 per cent of the votes cast in those Dargaville booths. Backing for Osborne - Sabin's successor - fell to 36 per cent on Saturday. There was a similar outcome in Kaitaia, where the deprivation index is highest.