There are specific standards for letterboxes in Australia

Like just about everything now there are standards for the design of letterboxes in Australia. These standards are found in AS-NZ 4253-1994

Here is an extract of the relevant part of the standards relating to letterboxes courtesy of mailmaster

STANDARDS AUSTRALIA/STANDARDS NEW ZEALAND

5 DESIGN AND CONSTRUCTION

5.1 General

5.1.1 Mailbox construction A mailbox shall be constructed to provide the following
characteristics:
(a) A reasonable life of the product.
(b) A product that is safe to use, and prevents cuts or injuries to the user.
(c) Be weather-resistant to protect the contents from rain, direct sunlight and other
elements of inclement weather. This would normally be provided by the use of a
weatherhood.

NOTE: It is recommended that some facility for drainage be incorporated into the design.
The mailbox shall be constructed so as to form an internal space for the purpose of receiving
and holding mail. The minimum dimensions of this internal space shall be 160 mm × 330 mm
× 230 mm. Except for any door-fastening mechanism or any internal weathershield, or both,
the minimum internal space shall be clear and unobstructed.

NOTE: It is recommended that a separate aperture, box or tube, as an adjunct to the mailbox, be
fitted to accept newspapers and other non-mail items. Mail should not be delivered to this second
aperture.

5.1.2 Front of mailbox The front or aperture side of the mailbox may be on the 330 mm
or the 230 mm long side, depending on the orientation of the mailbox.

5.2 Aperture for receiving mail The mailbox shall have an aperture in the front for
receiving mail as follows:

(a) Size of aperture The dimensions of the aperture shall be 230 mm × 30 mm or
330 mm × 30 mm, depending on the face on which the aperture is located.
(b) Lower edge of aperture The position of the aperture on the face of the box shall be
as follows:

(i) For horizontal apertures, the lower edge of the aperture shall be a minimum of
130 mm above the base of the internal space as defined in Clause 5.1.1.
(ii) For vertical apertures, the lower edge of the aperture shall be a minimum of
40 mm above the base of the internal space as defined in Clause 5.1.1. Mail
should not be able to be easily removed through the aperture.

5.3 Weather protection devices Weather protection devices, such as weatherflaps,
weatherhoods and weathershields, are acceptable provided they conform to the following:
(a) They shall not be closed by any spring force or other mechanism other than their own
weight.
(b) A weatherflap shall move inwards to allow receipt of mail.
(c) They shall not reduce the clear aperture opening, as defined in Clause 5.2, to less than
30 mm.

5.4 Access door An access door shall be provided to allow mail to be removed easily.

5.5 Provision for street/unit number An unobstructed smooth surface shall be provided
on the front of the mailbox below the aperture to accept self-adhesive or other numbers.

6 PERFORMANCE REQUIREMENTS

6.1 General All mailboxes shall be capable of separately accepting the two templates, as
specified in Clause 6.2. The templates shall be able to pass through the mailbox delivery
aperture and be deposited wholly within the internal space of the mailbox. The templates
shall be able to be easily removed via the access door.

6.2 Templates Both templates shall be of rigid construction and of the following sizes:
(a) 229 mm × 324 mm by 10 mm thick.
(b) 162 mm × 229 mm by 20 mm thick.

7 MAIL SECURITY On all mailboxes, the minimum provision shall be the facility to
accept a locking device. Mailboxes should be designed so that the mail cannot be easily
removed through the delivery aperture.

APPENDIX A
POSITIONING AND NUMBERING OF MAILBOXES
(Informative)
A1 SCOPE This Appendix is intended to provide householders and landlords with
appropriate information to assist delivery officers in the delivery of mail, except when
designated as mandatory by an authority. Correct positioning and numbering of mailboxes
will assist in the accurate and efficient delivery of mail.

A2 POSITIONING OF MAILBOXES Individual mailboxes and groups of mailboxes for
residences or business premises should be located as close as possible to the footpath,
driveway or roadway boundary of the property, with the aperture facing the footpath or
roadway.

If the property has a driveway entrance from the roadway, mailboxes should be positioned
as near as practicable to the junction of the driveway and the roadway.

In all cases, access to the delivery aperture should be unhindered, with the removal of rubbish
and overhanging shrubs and trees.

Mailboxes should be positioned so that the aperture is between 900 mm and 1200 mm above
ground level and conveniently located so that the delivery officer does not have to dismount
from a bicycle or motorcycle to insert mail.

Groups of mailboxes for flats or apartments may be positioned at right angles to the footpath
or roadway and parallel to the driveway with apertures located within the height range of
600 to 1600 mm. Mailboxes positioned in this way should commence as close to the footpath
boundary as practicable, and provide adequate room for the delivery officer to park a bicycle
or motorcycle and place mail in the boxes safely.

A3 NUMBERING OF MAILBOXES

All mailboxes should be clearly labelled with the street number or flat or apartment number clearly displayed below the aperture.
Where there is a group of mailboxes, numbers should run in a logical sequence and the street
number should be displayed in a prominent position near the mailboxes so that it can be
easily read from the footpath.