A group of pro-Islamic State (IS) hackers have gone on a website defacement spree, breaking into the websites of more than 20 Australian small businesses.

Key points: United Cyber Caliphate leave pro-IS messages, pictures on websites of 20 businesses

United Cyber Caliphate leave pro-IS messages, pictures on websites of 20 businesses Businesses included wheel and tyre retailers, Mexican food catering company and natural herbalist

Businesses included wheel and tyre retailers, Mexican food catering company and natural herbalist Group had been plotting attacks for several weeks

The hacking group, which calls itself the United Cyber Caliphate, has hacked the sites leaving a message and a picture.

Most of the websites that were attacked were tyre and wheel retailers.

"In the name of Allah, we are United Cyber Caliphate. Obey Islamic State. Your system is fail. Islamic State #rules," the picture read.

It is unknown why the pro-IS group targeted the websites, or what it has against Australian tyre retailers.

Its motive is confused further by the fact that a Mexican food catering company was also attacked, in addition to a natural herbalist website.

Website owners contacted by the ABC this morning were surprised that their websites had been hacked, and said they would call their website managers immediately.

"Are you joking?" the receptionist of one non-profit group said when informed about the hack. The call was put straight through to the chief executive.

"Oh my god. F***," the chief executive said.

"I'll call the web guy now. Thanks for letting us know."

When the ABC reached out to the website administrator contractor, the man who answered the phone was exasperated.

"Of course I know about the hacking. I've been spending all my time talking to customers, rather than fixing the problem," he said.

The small businesses said they just wanted to get on with running their business. Getting hacked by a pro-IS group was an annoyance they did not need on a Friday morning.

But little did the small business owners know, planning for the attack had been underway for several weeks.

It had been under heavy discussion in Telegram IS chat groups, which began when three hacking groups merged to become the United Cyber Caliphate.

Previously the three groups — the Caliphate Cyber Army, Sons Caliphate Army and the Kalashnikov Team — acted more independently.

But on April 5, they announced in English and Arabic that they would merge to: "Expand our operations. To hit 'em deeper."

The new group thanked "almighty Allah and his grace" for the successful merger operation.

In follow-up posts on the encrypted Telegram chat, the group warned "#Australia" to "#get ready for the next #attack", which they promised would happen "#soon".