I've been told because of taxes etc in Australia its cheaper to buy round trip flights from outside the country. For example, in the case of going from Perth to Bergen, purchase the ticket in Bergen. How do you do that and is that a fact or fiction?
It's mostly fiction. It's hard to find a specific example where this would make a significant difference.
For example Bergen <-> Perth in mid February was about $1100 and Perth <-> Bergen was about the same with one decent option being about $40 cheaper.
There is always variability of flight pricing, so it's hard to do an Apples to Apples comparison. Prices depend a lot on season, day of the week, special events at destination or departure, convenience of the connections, supply and demand, competition and many more.
"Buying outside the country" doesn't matter much either. First you need to define what that actually is. Which country is the one you buy in ?
- Country of Departure airport: you can't change that unless you change your travel plans
- Country of residence: the airlines don't even ask about this
- Country of citizenship(s): only important in terms of visa and entry rules
- IP address of the computer you book from: can easily be fudged with a VPN
- Billing address of your credit card: may affect fees, but that depends often on the card
- Currency used for payment: may affect fees and is subject to fluctuation
Typically none of these makes much of a difference.