Hi riba_ri

davewil1964 is essentially right here.

What's happening is that Australia Post has a few different ways to buy postage, one being over the counter rates, the other common one for small business sellers is "MyPostBusiness", and the third for larger sellers (not relevant here) are negotiated contracts under eParcel, but you have to be huge to get a new one of them.

Retail over the counter rates with Australia Post for up to 1kg actual weight and up to 3kg actual weight with a "cubic" factor allowed for (ie package size) are the same Australia Wide. So if you're sending 500gms it's the same cost anywhere. And that's what eBay has charged in the past if you book postage through them, the retail rates.

Now with MyPostBusiness, all locations are divided into three categories, also based on where the sender is. These are:
A) Local and/or Same State Capital City
B) Other Major City
C) Rural and Remote
MyPostBusiness users then get a discount off the retail rate, depending on 
A) Which Zone it's going to (in relation to the sender) - bigger discounts for local and same state, not much discount at all for remote, in the middle for other major city.
B) What "Band" they are on, ie Discount level. Discount levels vary from 0 (Retail Rates) up to 5 (Quite good discounts). Most smaller sellers who use MyPostBusiness are on bands 1, 2 or 3, with larger sellers getting up to 4 or 5.

What eBay has negotiated with Australia Post is to offer the equivalent of Band 2 MPB rates when booking postage through eBay.

In one sense this is good - sellers who don't use MPB or don't quality for Band 2 can get Band 2 equivalent rates (ie cheaper postage) though eBay. Happy days.

However where they have done the wrong thing by sellers in my opinion, is that they are passing on these discounts to the buyers as well, if you use Calculated shipping. Given eBay charges Final Value Fees on Postage, a lot of sellers (me included) would charge full retail price for shipping, but then buy the postage cheaper, with the difference roughly covering the FVFs.

Now, if you use calculated postage, the buyer is getting the discount, and unless you are on a MPB level above 2, you're not getting any difference to help cover fees. So maybe more sales from lower postage, but you're up for the fees yourself. And also, anyone who doesn't use MPB and doesn't buy their postage through eBay, they might be significantly out of pocket if they just go and pay for postage over the counter. 

So essentially, cheaper postage for you to buy because there are now discounts in the system. But just make sure your buyers aren't getting a discount and then you don't, leaving you out of pocket, if how postage to the buyer is costed vs how you actually buy it.

There are a few other options for how you can manipulate what is charged to the buyer (they suggest flat rates, but those aren't always the best option for every products/seller) but that's beyond this comment.

Hope this helps!

The first is going rural, the second main metro to main metro within the same state.

The whole postage thing on ebay is hit and miss. It's been a mess. And unfortunately, I don't think they're going to change it. 

Just to add, it could be the different zones coming into play. 

Are  you using Australia Post site or the ebay postage labels?