Two monuments to colonial Australia have been vandalised in Melbourne’s Flagstaff gardens overnight.

Police said they believed machinery was used to pull down the Pioneer Monument, a sandstone obelisk erected in 1871 to commemorate the first European burial ground on the site.

The monument was smashed into pieces beside the plinth on Thursday morning and daubed with the words “land back” and “death to Australia” in red paint.

Police said a crime scene had been established around both monuments and the investigation is ongoing. Photograph: Ellen Smith/The Guardian

An inverted red triangle was also painted on the monument. The triangle appears in the Palestinian flag at a different angle, and to some is considered a marker of resistance, but it has also been a symbol associated with Hamas.

Another nearby monument, the Separation Memorial, was also vandalised with red paint, police said.

The Separation Memorial was erected in 1950 to mark the centenary of the formal separation of the Port Phillip colony, now known as Victoria, from New South Wales.

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A crime scene had been established around both monuments and the investigation is ongoing, police said in a statement on Thursday.

Police believed the incidents had occurred between 10pm on Wednesday night and 6am on Thursday morning.

The paint had been largely washed off the monuments and workers were removing the smashed masonry from the site just after 10am on Thursday.

Melbourne’s lord mayor, Nick Reece, said the council had increased security at statues and monuments across Melbourne in the lead-up to 26 January. Photograph: Ellen Smith/The Guardian

The Victorian premier, Jacinta Allen, said the incident was “a disgrace”.

“Victoria Police will investigate and I’m asking anyone with information to contact Crime Stoppers. The groups who continue to commit these crimes must face the full force of the law,” she said.

Melbourne’s lord mayor, Nick Reece, said the council was working closely with police on the matter.

“This sort of behaviour will not and cannot be tolerated in Melbourne,” Reece told 774 ABC Radio Melbourne on Thursday morning.

He described the unknown perpetrators as “idiots”.

“I mean, to deface and say ‘death to Australia’ across a monument which was actually celebrating a great step forward for democracy and giving people the right to a vote, and all these other great rights and benefits [that] came from Victoria becoming a colony, is just moronic.”

Statues commemorating Australia’s colonial history have been targets for vandalism in the past in Melbourne, with Australia Day on 26 January being a particular flashpoint.

On 24 January 2024, a statue of Captain Cook in St Kilda’s foreshore gardens near Jacka Boulevard was sawn off at the ankles, and the plinth had been spray-painted with the slogan: “The colony will fall.”

This year, Thursday 22 January had also been marked as a national day of mourning for the 15 people who were killed in the December terror attack at Sydney’s Bondi beach, in which two gunmen, allegedly inspired by Islamic State, targeted a Hanukah event.

“Even on a day about love and unity, these people can’t help but introduce hate and destruction,” Allan said, referring to the national day of mourning.

Reece said the council had increased security at statues and monuments across Melbourne in the lead-up to 26 January, with cyclone fencing and cameras in some areas, but that was not present at the Flagstaff monuments on Wednesday night.

“We’ll obviously be reviewing arrangements across the remainder of our collection and we’ll be stepping things up in the days ahead,” Reece said.