The Commonwealth Park colony is the largest it has ever been and has grown by almost 200 fold over the past five years, according to ACT Wildlife.

The change was visible on Sunday, when frightened bats poured into the sky during the Australia Day celebrations, alarmed by the fireworks and cannon fire.

Wildlife ACT bat

co-ordinator Kirstie Hawkins said the bats began breeding for the first time in Commonwealth Park in 2013 and there were now more than she had seen before.

The largest officially recorded number of bats in the gardens was 6010, in 2011.

Ms Hawkins said she believed the big increase was due to bats pouring into the territory to escape fires on the NSW south coast.

''Until about five years we only ever had about 30 bats seasonally, then cyclone Yasi came through and there was the flooding up in Queensland. The rain knocked all the blossoms off, so they had no food. They came up here in huge numbers,'' she said. ''They never left.''

The colony is predominantly made up of grey-headed flying foxes, which feed on fruit and eucalyptus buds and are classified as a vulnerable species.

The bats formed a maternity colony for the first time in 2013, as pregnant bats flew in to give birth around mid October. Previously it had been a scout colony, formed predominantly of males.

A spokeswoman for the National Capital Authority said the organisation was undertaking a five-year surveillance program of the bat population with the help of the Australasian Bat Society.

If the number of bats remains high or if it increases, the NCA may have to move them on.

''It's three years into a five-year survey … it [currently] looks like the number fluctuates so much it mightn't be a concern for us,'' the spokeswoman said.

''If we were seeing consistently high numbers it might be something we'd get advice from the Department of Environment on.''

Ms Hawkins said she believed the bats were of great benefit to the territory and hurt no one in their current state.

''They're very good pollinators, they're better than bees. They do no harm, they share your fruit and there really is enough to share,'' she said.

The National Capital Authority said their main concerns revolved around public safety, including bites and scratches and exposure to bat excrement.

RSPCA acting chief executive Jane Gregor said no positive cases of Australian bat lyssavirus, which any bat can potentially carry and is closely related to the rabies virus, had been recorded in the ACT. But she still advised residents who found bats not to touch them or handle them.

''Don't try and handle them if they get caught up in netting and things like that. Call us. We'll either send out someone who's vaccinated or someone from ACT Wildlife will come out and get them,'' she said.

Wildlife ACT president Marg Peachey said Canberrans needed to be very careful when putting netting on their fruit trees, as the bats could be badly injured by incorrectly tied nets. ''Only a few people in Canberra net their trees properly,'' she said. ''The net needs to be tight, so when bats land on it they can fly off again. Don't just drape it over the tree, because bats can get caught in it.''