Prime Minister Tony Abbott and his wife Margie have added flowers to the constantly expanding makeshift memorial in Martin Place.

The pair left bouquets before signing the condolence notebooks set up on a trestle table at the side of the shrine.

The visit lasted only a few minutes and they didn't speak to anybody before returning to their cars.

After they left, the steady stream of people leaving flowers and writing messages resumed.

There were corporate workers on their breaks, tourists who had just arrived in Sydney, parents who had travelled long distances on trains with heartbroken children. There were people of all faiths.

Such was the outpouring of grief, that throughout the day people waited in long queues to leave their flowers.

Apart from city traffic and the clock marking the hour, there was almost complete silence around the shrine, which formed just a block away from the Lindt Cafe.

Tears streamed down the faces of some of those watching and waiting.

Police had to continuously move the crowd back to make more room for the increasing bouquets.

Retired nurse Cat Delaney stood nearby for hours with a box of tissues, offering them out to those with red eyes.

"All sorts of people have been coming up to me for a tissue, a hug and cry," Ms Delaney from Newtown said.

"This is what Aussies do, we look after our mates."

Cathy Butera drove about an hour from her home in Austral to leave two bunches of flowers.

"One act of evil from one person has now created a chain reaction with thousands of acts of kindness from people all over the world," Ms Butera said.

At one point, a woman began singing Amazing Grace, urging the crowd to join her.

Malika Ali, from Liverpool, said her three children had watched the siege unfold on television and had made cards to leave at the makeshift shrine.

"My kids wanted to bring cards and I wanted to show the world that not all Muslims are the same," Ms Ali said.

"The sadness will always be there when you remember this."

Canterbury resident Elizabeth Murray said she was moved by how the community had come together to mourn.

"What I think we're all doing here is making a statement. We're a community and a nation coming together," Ms Murray said.

It was not only flowers that were left.

There was a box of Lindt chocolates and a poster that read: "We feel your pain. Unlike people, heartache doesn't discriminate".