Every international shipment needs a commercial invoice showing seller name and address, buyer name and address, description of goods, HS code, quantity, unit value, total value, and country of origin.
For UK shipments post-Brexit, you also need a customs declaration (CN22 or CN23 for postal, or a formal entry declaration for commercial freight). UK VAT is collected at the point of import unless you register for UK VAT.
For Australia, the A$1,000 low value threshold for GST changed in 2018. All goods imported for sale in Australia are now subject to GST regardless of value — sellers need to register for Australian GST.
A 3PL with international shipping experience will prepare documentation as part of the dispatch workflow. If your 3PL requires you to provide customs docs yourself, that is an operational risk at scale.