Distance is part of every price — the same parcel costs more crossing the country than crossing a city — so the warehouse a quote ships from matters as much as where it is going. You choose how that warehouse is picked, on Shipping, under Ship-from origin.
Let Shopify decide — the default
Shopify knows which of your locations actually holds the stock for an order, and that is knowledge no routing table can match. On this setting, every order is priced from the location Shopify names. One warehouse or five, nothing to configure — this is the right answer for most stores.
Use my postcode rules
With more than one warehouse, you can route by destination instead: your rules say which warehouse serves which postcodes — orders heading to 6000s ship from Perth, everything else from Melbourne. Each order is priced from the warehouse your rules pick for its destination.
Your rules also work as the safety net on the default setting: when Shopify's answer can't be used — a location that can't ship, or one outside Australia — your rules decide instead, and the quote's log entry says that is what happened rather than passing it off as routing you chose.
The one hard requirement
A warehouse can only quote if its Shopify location has a street address in Australia. Without one there is nowhere to price from, and rather than guess, TellShip quotes nothing — the fourth cause in My rates aren't showing at checkout.
Try it before an order does
On Test rates, the warehouse starts on "Use my postcode rules" — chosen automatically, the way checkout would — and you can override it to quote the same parcel from another warehouse. The Rate factors panel names which warehouse priced the result, so a routing rule is checked in one quote.