A team of researchers from the University of Sydney will return to the area in early April to administer a second vaccine after 'disappointing' results from the original vaccine in a trial in September 2020.

In January, the university's veterinary pathologist Professor Mark Krockenberger said the original vaccine had reduced the amount of chlamydia the vaccinated and infected animals had shed, but it had not stopped the transferral of the potentially deadly disease, which renders females infertile.

The second trial was slated for March, but the researchers held off because of the torrential rain and flooding in Sydney and many parts of NSW.

"We know if it rains, we can't get to the koalas, so we just took a bit of a raincheck," zoologist Dr Valentina Mella said.

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Dr Mella said hopes to catch more than 50 koalas for the trial, including 35 animals, which received the first vaccine. The vaccine will be administered to sick and well koalas to both combat and prevent the disease.

"It's a bit like a booster. I feel like nowadays, everyone is familiar with vaccines and boosters," Dr Mella said.

"Everybody feels more confident about this one compared to the first one."

The team usually bases its work at Robert Frend's property, Dimberoy, and surrounding properties, but Dr Mella said they had to expand their research area to Breeza because of dwindling numbers.

"Every time he [Mark Krockenberger] went for a trip for radio tracking, they always found a couple of animals dead, and they're old animals, and we know they're going to go, but it's always sad when they do," she said.

"We still haven't seen young ones to replace the old ones, which is what we're really worried about, which is why we have to extend the study area."

Dr Mella said this did make quantifying results more complicated, but they had no choice.

"You want to make sure all the animals you're working with are exposed to the same environmental conditions, but if there's none, you have to expand to somewhere else," she said.

The group will visit Dimberoy first, then move onto the Clifts property on the Liverpool Plains. They are somewhat familiar with their property, The Dip, after visiting in 2018 to check on koalas that were close to where the site earmarked for the then-proposed Shenhua Watermark mine.

"[The Clifts] said they don't see as many around, so we don't know what we're going to find," Dr Mella said.

"We're hoping to find young, healthy animals ... because at the moment, we're testing a vaccine on a population of animals that is quite old, so the results that you get are always limited to that age group. If we found young, healthy animals, maybe their results would be different."

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