The service to be held at the Royal Exhibition Building, Carlton Gardens, will honour those who lost their lives in the attack, their families and those who were injured and affected.

It will also be an opportunity for those people to come together with first responders.

The service will also be live streamed on the government's Bourke Street website.

The memorial will be held from 12pm to 1.30pm. Doors will open at 10.30am, but guests are encouraged to arrive no later than 11.30am.

Members of the public are encouraged to attend, however they are not required to register.

Due to arrangements at the service, there will be no opportunity to lay flowers, but the Bourke Street collection of tributes - including cards, photos, artwork, toys and the book of condolences - left outside the GPO in the mall in the days after the incident can be viewed privately at the State Library of Victoria by emailing manuscripts@slv.vic.gov.au or phoning Australiana Heritage Collections on (03) 8664 7009.

The floral tributes were mulched and spread across the three victims of crime memorial sites in Melbourne.

As a mark of mourning and respect, the Australian flag will be flown at half-mast all day on Tuesday from government buildings. Other organisations are welcome to participate.

For more information, including door opening times, go to www.vic.gov.au/bourkestreet.html.