by tl;dr
For letters and documents less than 250g, it's cheapest to send in your own envelope ($3.40 up to 125g, $5.10 up to 250g).For letters and documents over 250g and under 500g, you can send in your own envelope for $8.50 or use a prepaid B4 envelope for $8.80.
If you need tracking then consider using a $9.40 prepaid C4 envelope with tracking or an $9.90 Registered Post B4 prepaid envelope.
Express post is the quickest option and costs $11.05 for a prepaid B4 envelope.
Regular letters
Australia Post define a large letter as up to 360 x 260mm in size and a maximum of 20mm thick. The weight limit for letter rates is 500g.
If you use your own envelope, you will pay based on weight:
Up to 125g - $3.40
Up to 250g - $5.10
Up to 500g - $8.50
However, in some cases it may actually be cheaper to use one of Australia Post's prepaid envelopes. For large letters, these now come only in B4 size (C4 were discontinued in July 2025), and you can send up to 500g.
B4 (353 x 250mm) - $8.80
Source: https://auspost.com.au/sending/send-within-australia/compare-letter-services/regular-letters-cards and https://auspost.com.au/business/shipping/satchels-and-packaging/prepaid-satchels-and-envelopes
Domestic letter with tracking
If you need tracking, Australia Post offer a prepaid C4 envelope (329 x 229mm) that includes tracking and proof of delivery for $9.40. These have the advantage of being a sturdy cardboard instead of paper so will better protect the contents and are unlikely to be folded. Being cardboard, though, they are heavier (about 70g) and will use up some of the 500g weight limit.
Source: https://auspost.com.au/sending/send-within-australia/compare-letter-services/letter-tracking
Registered Post letters
If you need tracking for something larger than will fit in a C4 envelope, or if you need a signature on delivery, then there is a B4 size (353 x 250mm) Registered Post envelope available for $9.90. These also have a 500g weight limit.
Source: https://auspost.com.au/sending/send-within-australia/compare-letter-services/registered-post-letters
Express Post
Express Post for large letters is available in one size only – B4 – which costs $11.05.
Express post includes tracking and delivery is next business day to major cities, although this is no longer guaranteed.
Source: https://auspost.com.au/sending/send-within-australia/compare-letter-services/express-post-letters
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