Adelaide Zoo was forced to close Friday after would-be thieves blew up an ATM at the gift shop.

A security patrol found the front of the ATM on Plane Tree Drive had been badly damaged after responding to an alarm about 3:30am.

Explosives experts examined the area and the zoo was shut as bomb technicians investigated a red liquid on the ground.

Sergeant Phil Clague said it is believed the liquid was a modified and volatile substance that is used in the industrial sector with hardened plastics.

But he said the ATM was able to withstand the blast, and the culprits fled empty-handed.

"ATMs are basically a safe with a computer on the front so they are built to withstand a significant attempt to gain entry to them," he said.

"I've been told this is the first time that this kind of explosive substance has been used in Australia."

Students were staying at the zoo overnight but police said they did not hear the explosion.

Booked groups were initially allowed to enter the zoo, but were eventually evacuated.

Adelaide Zoo to reopen Saturday morning

Zoos SA chief executive Elaine Bensted said its doors would reopen on Saturday morning under its usual opening hours schedule.

It followed advice from emergency services crews that there was no longer a safety risk to the public.

Ms Bensted said no animals were injured or harmed during the incident.

"However, animals close to the exclusion zone during the police investigation were moved to their night quarters as a precaution," she said.

The retail shop will remain closed over the weekend as the ATM machine is repaired and police continue their investigation.

"It's incredibly disappointing that someone would not only conduct a crime such as this, but also on a conservation charity," Adelaide Zoo's Russell Schrale said.

"This is going to cost the zoo money, which takes money out of our conservation programs.

"There's some life choices that [the culprits] haven't made which are particularly good."

Police are seeking anyone who may have seen or heard anything to come forward.