Alexis Carey, who attended Wade High School and worked at The Area News as a journalist from July 2010 to June 2011, works for Mamamia in Surry Hills.

"We can clearly hear police sirens from our office and we have had our eyes glued to the television since this horror started," Ms Carey said.

"It's hard to describe what the feeling is like in Sydney today. We are all in shock and literally feel sick knowing that this is unfolding in our beautiful city - within walking distance from where we are now."

Ms Carey expressed sadness at what this would mean for Sydney residents.

"It is an awful feeling to realise that we are no longer safe in what used to be one of the safest countries in the world," she said.

"I have had work-related meetings at the Lindt Cafe and have friends who work in the area. My first thoughts were to them, but luckily the are all OK."

Ms Carey said her thoughts were with the hostages and their families.

"I can't even begin to imagine what they are going through now and all we want is for them to get home safely today."

Sydney's CBD is in lockdown as police negotiate with a gunman who stormed into the busy Lindt Chocolat Cafe in Martin Place on Monday morning and took customers and staff hostage. 

In extraordinary and terrifying scenes, hostages could be seen through the cafe's windows with their hands held in the air, while heavily armed police surrounded the building.

A number of hostages were being forced to hold an Islamic flag against the window shortly after the siege began at 9.45am.

Thousands of workers across the city have been sent home early and some of the city's major buildings evacuated.

They include the Opera House, the State Library, Channel Seven, the NSW parliamentary executive offices, the NSW Supreme Court's criminal courts, the Downing Centre, and several city legal chambers.

An unknown number of staff and customers are being held at gunpoint inside the cafe. It was earlier reported that 20 hostages were being held.