"To see the crowd here in Martin Place, there was not a spot left open to be standing or sitting," President of RSL NSW Ray James told AAP in the early hours.

"There's not one person in Australia that doesn't know someone, or is a relative of someone who has worn the uniform."

He said the size of the crowd at the dawn service, particularly younger people, showed the Anzac spirit was alive and well in Australia.

Standing with Mr James beside the Cenotaph was Gary Charlesworth, who served for six years before becoming a member of the NSW Police Force.

Mr Charlesworth received military training from the RSL president and said he was moved to see the service filled with families.

"The last two years have been a little bit haphazard, but you could feel that there were tears shed in the crowd," he said after the ceremony, which was marked by patchy rain.

"It's good to see people feeling that community the Anzac spirit brings."

David Young, a serving member of the Australian Navy, told AAP he had attended the dawn service to educate his children about his military experience.

"Being in the Navy for 24 years, this is a chance to remind the kids of the history behind why I serve," he said.

"I've lost guys that I've served with and it helps to explain what I've done in the history of my career."

Cameron Barnett, 24, from the Scouts told AAP many former members of the youth group ended up going to war, and Anzac Day was of major significance to the organisation.

The Scouts were heavily involved in Anzac commemorations, he said, and many held umbrellas for veterans throughout the service.

NSW Premier Dominic Perrottet arrived under an umbrella with one of his daughters, and read the poem Salute by Sydney Napier.

NSW Opposition Leader Chris Minns also attended, and in a statement after the ceremony called for a new memorial to honour all those who served in campaigns in Iraq and Afghanistan.

Senator Hollie Hughes represented Prime Minister Scott Morrison, and Sydney MP Tanya Plibersek represented Labor leader Anthony Albanese.

Delivering the dawn address, Major General Matthew Pearse said it was a day to give thanks to all veterans "for their service, their sacrifice and their resilience".

Later in the morning, crowds lined Elizabeth Street for the Anzac Day march, during which a Royal Australian Air Force flyover broke the sound barrier.

Three-year-old Eleanor clutched a framed photograph of her grandfather, Thomas O'Brien, who served in New Guinea and Borneo during World War II.

Her father John O'Brien told AAP he was one of many veterans' sons who liked to keep the memory of their fathers alive.

"If it wasn't for them doing what we did, we wouldn't have what he have," Mr O'Brien said.

His father, who served for four years after joining at 18, was a "very honourable man" who went on to live a quiet life. He died in 2001.

Police said there were more than 5000 people at the Sydney dawn service, and more than 9000 marched.

Other commemorations took place in regional NSW.

In Lismore, the epicentre of flood devastation in recent months, Mayor Steve Krieg paid tribute to the ADF for their critical role in aiding communities at the height of the natural disaster and in the recovery.

The RAAF also conducted flyover displays in more than 50 locations across NSW, including Newcastle, Wyong, Cessnock, Ourimbah, Woodburn, Grafton and Bega.

Australian Associated Press