Zoos Victoria are among those working with researchers around the world by contributing their observations of several animal species, including tigers, gorillas, reptiles and butterflies.

All three of its popular Victorian tourist attractions - Melbourne Zoo, Werribee Open Range Zoo and Healesville Sanctuary - closed to visitors on March 24, reopened for five weeks, before closing again on July 8 when Victoria went into its second lockdown.

Chief executive Dr Jenny Gray said it was the first time all three locations had been closed in Melbourne Zoo's 158-year history.

"On very rare occasions, the local landmark has closed for a few hours to deal with weather damage, but has reopened the same day," Dr Gray said.

Considered essential workers, Zoo Victoria's keepers and animal behaviourists have maintained animals' feeding, cleaning, training and enrichment activities, despite the absence of onlookers.

"We can safely say that none of the animals are bored - visitors can indeed be a source of stimulation, but there's a lot more going on in their lives than visitors," Dr Gray said.

Zoo workers around the world have reported changed behaviour in animals in the absence of crowds, with primates at Phoenix Zoo in Arizona and Twycross Zoo in England observed to search for visitors.

A zoo in India reported its animals were loving the quiet spell, while rhinos and giraffes at Orana Wildlife Park in New Zealand continued to show up for their usual meet-the-visitors appointments, despite their being none.

Canberra zoo owner Richard Tindale said during its two-month closure some animals required extra attention from staff in the absence of visitor interaction.

"They all fared well but seemed happy to welcome back zoo visitors," Mr Tindale said.

The National Zoo and Aquarium visitor numbers were down 58 per cent between January and May this year, while income from other sources such as tours and schools was down between 70 per cent and 83 per cent.

Despite the ACT's easing of social-distancing measures, visitor numbers were down 8 per cent on August 2019, while tours and schools remained down between 54 and 94 per cent last month.

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Mr Tindale said income at the Canberra zoo had dropped more than 35 per cent last month, while Zoos Victoria reported a 75 per cent reduction in its usual revenue since March.

NSW has not been exempt from zoo troubles. A western suburbs Sydney Zoo which opened in December was forced to close just 109 days later.

"With bushfires, flooding, and of course the coronavirus pandemic, it has been an incredibly difficult start for Sydney Zoo," a spokesperson said.