This depends on which species you are hoping will use the nest box. While some birds are not fussy, others will refuse to use a tree hollow or nest box that is too low.
On my property, gliders and possums are not fussy about nest box height, they’ll take any low (3 m) to high (7 m tested so far) nest box that is suitable for their needs. I’ve only just installed nest boxes to 20+ meters in height, so am yet to see which species will use these ones (May 2019).
However the local Eastern and Crimson Rosellas were not interested until the nest boxes were installed at or above 4-5 meters.
The local Galahs are only this year showing interest in our nest boxes, after I purchased arborist (tree climbing) gear and installed some nest boxes at heights between 10-20 meters.
Wood Ducks however may quack about this height, perhaps too high for their little ones to leap safely to the ground. Our successful Wood Duck nest boxes are installed at heights of 4-5 meters.
The best nest box installation height will depend on which species you are hoping will use a particular installed nest box. There also appears to be local preferences of hollow using species, that differ from location to location.
To know which species occur in your area, ask your local conservation groups, and also your state or territory National Parks and Wildlife Service for a list of locally occurring animals including birds and mammals.
Below is a list of native birds and mammals that use tree hollows, and the height range that they have been found to use tree hollows. There will be some variation between regions, such as regions with many tall trees – preferred nesting heights will be higher, compared to regions with short and stumpy trees – preferred nesting heights will be lower. For example I live within a Scribbly Gum woodland, where the trees are quite short even at maturity. Our local animals are probably accustomed to short trees, while in regions with taller trees, expectations will be for much higher nesting hollow equivalents.
Use this link to look up any bird species listed below to find out more about it: http://www.birdlife.org.au/all-about-birds/australias-birds/find-a-bird
Birds:
Australian Kestrel: 6 m
Australian Owlet-nightjar: 5-30 m
Australian Ringneck: 1-16 m
Australian Shelduck: 2-30 m
Barn Owl: 5-30 m
Barking Owl: 2-35 m
Baudin’s Black Cockatoo: Unknown
Boobook Owl: 3-30 m
Blue Bonnet: 0.6-5 m
Blue-winged Parrot: 3-24 m
Bourke’s Parrot: 0.9-8 m
Budgerigar: 0.2-18 m
Carnaby’s Black-Cockatoo: 2-10 m
Chestnut Teal: 1-2.5 m
Cockatiel: 1-11 m
Cotton Pygmy-Goose: 4.5-21 m
Crimson Rosella: 4-23 m
Dollarbird: 6-35 m
Double-eyed Fig-parrot: 7-25 m
Eastern Rosella: 4-30 m
Eclectus Parrot: 6-35 m
Elegant Parrot: 3.5-46 m
Forest Kingfisher: occasionally uses tree hollows
Galah: 5-19 m
Gang-gang Cockatoo: 20-30 m
Glossy Black-Cockatoo: 5-28m
Green Pygmy-Goose: unknown
Green Rosella: 2-40 m
Grey Teal: 3.5 m
King Parrot: 6-25 m
Kookaburra: 5-60 m
Lesser Sooty Owl: 10-45 m
Little Corella: 1.5-11
Little Kingfisher: occasionally uses tree hollows
Little Lorikeet: 6-18 m
Long-billed Corella: 1.5-25 m
Major Mitchell’s Cockatoo: 2-19+ m
Masked Owl: 3-30 m
Mulga Parrot: 0.6-24 m
Musk Lorikeet: 3-8 m
Northern Rosella: Unknown
Orange-bellied Parrot: 4-40 m
Pacific Black Duck: 3 m
Paleheaded Rosella: 4-23 m
Pardalote (various species): 5 + m
Pink-eared Duck: 0-5 m above water
Peregrine Falcon: 9 m
Powerful Owl: 10-40 m
Princess Parrot: unknown
Purple-crowned Lorikeet: 1.5-12 m
Rainbow Lorikeet: 3-30 m
Red-backed Kingfisher: occasionally uses tree hollows
Red-capped Parrot: 5-16 m
Red-cheeked Parrot: 15-20 m
Red-rumped Parrot: 0.5 20 m
Red-tailed Black Cockatoo: 1-15 + m
Red-winged Parrot: 5-11 m
Regent Parrot: 5-30 m
Rufus Owl: 8-40 m
Sacred Kingfisher: 0.5-35 m
Scaly-breasted Lorikeet: 3-20 m
Scarlet-Chested Parrot: 1.2-8 m
Sooty Owl: 10-50 m
Sulphur-crested Cockatoo: 1-35m
Superb Parrot: 7-32 m
Swift Parrot: 6-35 m
Treecreeper (all species): 3-5 m
Turquoise Parrot: 0.2-20 m
Varied Lorikeet: 1-9 m
Western Corella: 3-14 m
Western Rosella: 4-13 m
Yellow-billed Kingfisher: 3-15 m
Yellow-tailed Black-Cockatoo: 5-56 m
Mammals:
Brushtail Possum: 3-5 m
Feathertail Glider: 2 m
Microbats: 3 m
Ringtail Possum: 3-5 m
Squirrel Glider: 4-8 m
Sugar Glider: 4-8 m
References: Nest boxes for Wildlife: A Practical Guide (A. Franks & S. Franks, 2011); Tree Hollows and Wildlife Conservation in Australia (P. Gibbons & D. Lindenmayer, 2002).