But for almost a year, a website for the fake council had been informing residents of Hornsby and Ku-ring-gai their local councils had been abolished and replaced with the new super council.

In a bizarre twist, the fake council - North Shore Council - was cooked up by the communications department of Hornsby Shire council, and unleashed upon the internet even though no such merger had yet taken place between the two councils.

By Monday morning, the website had been taken down, leaving Hornsby Shire council red-faced and claiming it was unaware the site was live until it received media inquiries that morning.

"The website was not meant to be public. That was a simple mistake which has now been rectified," a council spokesman said.

The domain, northshorecouncil.com, was registered by Hornsby Shire Council last year, shortly after the NSW government announced in May their intention to amalgamate Hornsby and Ku-ring-gai councils.

The new council's name, a decision which ultimately resides with the NSW government, was "simply a guess" by Hornsby Shire's PR department.

"Hornsby Council has no information that North Shore Council will be the name of the new merged council," the spokesman said.

"This was simply a guess by our communications team so that a domain name could be registered and work commenced on drafting a site."

The website remained live, declaring the existence of the new council even as Ku-ring-gai council launched legal action to fight the forced merger last year, indefinitely prolonging the creation of a new merged entity.

It is understood that neither the mayor nor the executive of Hornsby Shire council were aware of the existence of the website until Monday morning.

However, the issue has intensified hostilities between the two councils, which have fermented around Ku-ring-gai's vehement opposition to the merger and Hornsby's support for the amalgamation to occur as quickly as possible.

Ku-ring-gai council confirmed on Monday it was not consulted about the website, and was only made aware of its existence after receiving inquiries from confused residents as to whether the merger had already occurred.

"This action by Hornsby council is another appalling example of the arrogance with which they have approached the merger issue all along," Ku-ring-gai mayor Jennifer Anderson said.

"To make this website live shows a level of disdain for Ku-ring-gai and does not bode well for any future merger."

Cr Anderson she had lodged a formal complaint with Premier Gladys Berejiklian over the issue, and said council lawyers would examine whether it had potentially prejudiced the council's Supreme Court of Appeal case.

But Hornsby Shire defended the move as "just one example of the preparations" the council was undertaking to prepare for the merger.

"If the merger goes ahead, we want to be in a position to deliver the full level of service to ratepayers from day one. This includes keeping everybody fully informed," the spokesman said.

The website debacle is the latest example of bad blood between the two councils, foreshadowing the potential difficulties which will confront a newly merged entity.

Earlier this year, Hornsby Shire mayor Steve Russell accused Ku-ring-gai councillors of "clinging to the selfish fiefdoms they've carved out using ratepayers' money" by resisting the merger.

It prompted a stern response from Cr Anderson, who maintained ratepayers were opposed to "propping up Hornsby" with Ku-ring-gai's assets.