How do you indicate a unit number when writing the postal address of snail mail?

Australia Post's Addressing guidelines does not mention how to indicate unit numbers.

Wikipedia states that for a flat or apartment, you should have the unit number, then a forward slash, then the street number. However, it does not have a citation for that.

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    I think this is Too Localised for an English Learners site. FWIW, I would put the flat/unit number on a separate line (as "Flat 3", or "Unit 6a", etc.) after the name, and before the "second to last line" containing the "[building] number and name of the street" as mentioned in your link. – FumbleFingers Apr 22 '13 at 13:01
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    6a/100 Main Street, U6a/100 Main Street, Unit 6a/100 Main Street. All of those are acceptable and all are equally likely to end up getting lost in the post. – mcalex Apr 23 '13 at 11:31
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    This post has received 10K views. – Andrew Grimm Oct 8 '16 at 6:49
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    @AndrewGrimm I have found a very useful document from Australia Post: auspost.com.au/media/documents/…. It might be a little old according to its filename, but it clearly describes how to format unit numbers. It should be like U 2 17 Jones St or 2/17 Jones St. Please note that it is preferred that there are 2 spaces between the unit number and the street number when using the first format. – Domon Dec 3 '16 at 0:45
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    This still shows up quite highly in search engine results when searching this question, for a couple different wordings of it. Thought I'd pop the answer I found from Auspost after a bit of digging. Page 3 of this document. If anyone knows a more appropriate part of StackExchange for this question, feel free to throw the link there as an answer for me :) – FreelancerJ Aug 23 '19 at 1:23