Television New Zealand's late-night news show Tonight has failed to rein in the ratings lead of TV3's Nightline, despite four months of tinkering.

Tonight, hosted by Eric Young and Kate Hawkesby, was launched on January 19 as a more relaxed version of the 6pm One News.

Ties were discarded and DVD and film reviews were introduced, as magazine-style stories ran alongside informal interviews and reworked versions of stories previously read by Judy Bailey.

After four months, Tonight continues to lag behind its competitor by an average of one rating point in TV One's 25-54-year target audience and by almost two points in TV3's 18-49-year target market.

According to ACNielsen ratings for 25-54-year-olds, Tonight attracted 4 per cent of the total potential audience and 20 per cent of those watching at that time on Wednesday night, while Nightline drew 7 per cent and 26 per cent, respectively.

But the ratings also show Nightline's audience is highly variable. While Tonight gently ebbs and flows between 3 per cent and 6 per cent, Nightline can drop from 9 per cent down to 3 per cent on consecutive nights.

TVNZ head of news and current affairs, Bill Ralston, accepted Nightline has rated consistently well for years and said the nature of TV One's programming would make the gap difficult to peg back.

TV One shows a large proportion of British programmes, usually provided by the non-commercial BBC, which run longer than a normal commercial hour. Depending on the previous show, Tonight can begin screening at anytime between 10.30pm and 10.45pm.

Mr Ralston said that as a result, the TV3 news was often first cab off the rank and more likely to pick up floating viewers. But he said the Tonight news team were encouraged by results which showed they were rating higher than its predecessor, Late Edition, was at the same time last year.

In contrast, Roger Beaumont, TV3's director of marketing and communications, preferred to see Nightline as thriving because it had maintained a stable lineup and style for about five years.

He said TVNZ had regularly played around with its late news, which had turned some viewers away.

Mr Beaumont did not agree timing was a factor.

"Nightline has been a long-term success for us. It is well-established and has attracted a regular audience who have become comfortable with the format.

"From our perspective, the battle between the channels has been a steady, consistent war, but every year we're on air we seem to be gaining audience."