However, the return ticket for premium economy is about the same price?
Is is worth to redeem the Qantas points for one way?
International fares tend to be priced (when paying cash) so that the cost of a one-way fare is only somewhat cheaper than a return, rather than being half the cost of a return. (In contrast, domestic fares are generally priced so that a return ticket costs the same as two one-ways.)
It's a frustrating situation and it makes redeeming points for only the outbound (or only the inbound) journey very poor value, as you're basically paying close to double for the flight going the other direction.
I faced this a few months ago when I managed to find an award flight to the UK but no award flight coming back (for the dates I wanted). It would have cost me almost as much to buy a one-way ticket from the UK to Australia as an Australia-London-Australia return fare.
In your case you'll probably be better off trying to find award flights for the return trip, as the cost of award flights per leg remain the same regardless of whether you're going one way or return. In other words, if a flight - excluding the cash component - costs 200,000 points return, it'll cost 100,000 points one-way. In contrast an international cash fare might cost $2,000 return but $1,800 one-way. (These numbers are made up to illustrate the point.)
If you don't have sufficient points for two return flights, you might want to look into paying for one of your tickets in points and the other in cash. It may end up being better value.
You can mix and match classes for award flights by making separate bookings, ie fly out in Business and come back in Premium Economy. It's trickier when paying cash. You might need to settle for being in one class for both the inbound and outbound.