Prime News out-rated its TV3 competitor last night, as the MediaWorks' channel lost nearly half its audience in two weeks.
3 News drew an audience of 166,000 viewers last night - 13,000 less than Prime News, which screens half an hour earlier at 5.30pm.
Two weeks ago, 3 News attracted nearly double that figure, with 314,000 viewers tuning in.
Media commentator Martin Gillman says the numbers are surprising and will be the cause of serious concern at MediaWorks, which is currently examining its news and current affairs programming in a bid to maximise revenue.
"News makes up a significant part of the schedule and is too important for them to lose out so badly," he said.
The ratings slide coincides with changes to TV3's Sunday night schedule, which saw it move to a reduced half-hour news bulletin, ahead of the weekly current affairs programme 3D.
The new format debuted on May 24 to reasonable ratings, but viewers have failed to stay with the line-up. Last night's episode of 3D saw just 160,000 viewers tune in.
Gillman says it's possible some viewers have tuned out following MediaWorks' decision to axe Campbell Live and the company's perceived lack of commitment to current affairs.
"MediaWorks obviously feel comfortable experimenting but they've had too many failures lately. They'll be thinking long and hard about what to do now."
Meanwhile, TVNZ's One News drew a massive 802,000 viewers last night, making it one of the year's top rating shows.
Traditionally, Sunday night delivers the highest viewership figures, making last night's loss even more concerning. However, Gillman warns against looking at just one day's figures.
"They have a broader issue around their news ratings. Yesterday was a low point for them but it's definitely an ongoing problem."
Ironically, Prime News' ratings success comes after MediaWorks took over production of the bulletin in March.
3 News: 166,210
One News: 802,400
Prime News: 178,600
3 News: 212,640
One News: 640,220
Prime News: 164,450
3 News: 313,520
One News: 805,820
Prime News: 184,140