Brush-tailed possums prefer to reside inside roof crevices. Because they are territorial, removing one will not resolve your issue because the new possums will quickly move into the area. Possums that have been relocated will not be able to survive in a new area.
This is why you should move them back to their original location, away from your roof. You can browse https://emersonsenvirocare.com.au/possums/ to contact the best possum removal firm in Sydney.
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It is important to first confirm that it's a possum and not a mouse or rat. Sprinkle flour around the manhole and examine the footprints this will help determine whether it is possums or not.
After that, create another home for the possum to live in. Invite the possums to look around their new surroundings by placing pieces of fresh fruit inside or around their nesting boxes.
Find out the method by which the possum moves into the roof by watching it at dusk as it leaves to hunt.
Dispel possums by spraying your roof with quassia chips (from Hardware stores) as well as naphthalene or camphor (Note: don't apply a mixture of both).
When you're sure that the possum has settled into the new house then block the entrance to the roof by putting up a one-way entrance.
If you're unable to complete these tasks by yourself, you can take the help of an experienced possum catcher or release operator to complete the task for you.