Seven's announcement follows Nine's move to a national one-hour bulletin at the start of this year, with A Current Affair moving to 7pm.
Nine's extended bulletin has lifted its ratings in the 6pm to 7pm slot, though Seven's long-running soapie Home and Away now comfortably beats A Current Affair at 7pm.
Last year, Nine won the 6pm to 7pm slot in Sydney, Melbourne and Brisbane while Seven claimed victory in Perth and Adelaide.
Channel Seven has dumped Today Tonight in Sydney, Melbourne and Brisbane, replacing the tabloid current affairs show with a one-hour news service.
But the program has been spared the axe in Adelaide and Perth, where it still wins the 6.30pm time slot against arch-rival Nine.
Seven will launch its extended news tonight. All Today Tonight staff on the east coast have been redeployed to make "news investigations and features" for the bulletin.
Seven's switch – the latest tactic in the long-running trench warfare between Australia's two biggest networks – comes as no surprise. Rumours of a one-hour bulletin on the east coast have been circulating for more than a year, and were intensified by the departure of Today Tonight host Helen Kapalos last month and Seven's one-hour bulletin on the Australia Day public holiday.
Yet last week, Seven's programmer Angus Ross told the TV Tonight website: "We're experimenting in the [6pm to 7pm] hour as Nine is, but we're not going to a one-hour news."
A Seven spokeswoman confirmed to Fairfax Media that no Today Tonight staff will lose their jobs. Instead, they will report to the program's executive producer, Max Uechtritz, who now reports to Seven's director of news, Rob Raschke.
Today Tonight staff will join the news teams within their city and produce longer reports and investigative features for the extended bulletin.
In an email to all Channel Seven staff this morning, network boss Tim Worner said: "This move is about building and investing in Seven's leading news coverage. In recent times we have increased our level of news across the day with great success. We plan to do even more in the future.
"We see this as an exciting opportunity to serve our news audience in a better way.
"I want to take this chance to pay tribute to Today Tonight and the TT team on the east coast. Over many years, and as recently as last week, the program has broken big stories, kept consumers informed and held people to account."
Worner said he will personally brief staff later this morning to answer their questions.