Data showing increases in the electorates of nationalist-held constituencies and a decline in Belfast could bring fresh pressure for another boundary review in the North.
The figures, produced by the Electoral Office, and seen by The Irish Times, highlight growth in voter numbers west of the Bann and in mostly nationalist areas.
The census report is not expected until later this year, but an outline report may be made available to the Assembly before it rises for the summer recess in early July. That report could confirm that significant demographic changes are under way.
The size of the Catholic population, roughly one-third of the total after partition, could now be around 45 per cent or higher.
The data, comparing electorate changes between 1996 and 2002, show a population drift from Belfast to outlying areas. However, the increase in nationalist-held constituencies is much more marked than in those represented by unionists.
There are SDLP or Sinn Féin MPs in the first five constituencies with the largest electorate growths.
Some unionist areas have also seen rises but these have been around 5 per cent and below, rather than the figures of 7-8 per cent recorded in nationalist seats.
The sharpest rise was found in Mr Eddie McGrady's South Down seat which had an electorate of 69,035 six years ago and 74,935 this year, a rise of 8.55 per cent.
The next largest increases are in Mid-Ulster, held by Mr Martin McGuinness; Foyle, held by Mr John Hume; West Tyrone, held by Mr Pat Doherty, and Fermanagh-South Tyrone, captured narrowly by Ms Michelle Gildernew at the last Westminster election nearly 12 months ago.
The only nationalist seat dwindling in size is Mr Gerry Adams's West Belfast, where the electorate slipped from 61,344 to 58,901, a fall of 3.98 per cent.
All other declines are noted in unionist-held seats, the largest being in North Belfast which was gained by Mr Nigel Dodds of the DUP last June. It decreased to 59,653 from 65,411, a fall of 8.8 per cent.
Northern Ireland used to have 11 Westminster seats, giving it a constituency electorate average well above that in Britain. This was addressed in the mid-1980s and the total was increased to 17. However, continuing demographic change necessitated the creation of another seat, West Tyrone, in time for the British general election in 1997.
The current 18 seats are used for Westminster first-past-the-post elections and for Assembly elections, when they are all treated as six-seater constituencies under proportional representation.
At the last major boundary review there was significant pressure to cut Belfast's Westminster total from four to three. This was resisted at the time, but with population decreases continuing, there could well be renewed pressure for the city to lose a seat.
The electorate per constituency average is now 66,580, but some seats such as the Rev Ian Paisley's are much higher than this. The DUP leader now has the largest constituency with 75,635 electors and all the nationalist seats, with the exception of West Belfast, are above the average and rising rapidly.
The change in demographics is also clear in statistics for the North's 26 district council areas.
The Belfast City Council area has slipped by 8.71 per cent, the Castlereagh borough - which includes much of the city's eastern and southern suburbs - has fallen by 1.46 per cent and the neighbouring North Down borough has fallen marginally.
However, both Banbridge, Co Down, and Ballymoney, Co Antrim, are witnessing electorate rises of 9.56 and 8.71 per cent respectively. These are strongly unionist areas.
District council areas with nationalist electorates all report growth of up to 6.35 per cent.