A couple needs $295,000 between them, which is $20,000 less than a single buyer, even with two people's living costs to cover instead of one. The reason is tax, not anything to do with the loan.
A single earner on $315,000 pays $113,920 including the Medicare levy and takes home $201,080. Split $295,000 evenly and each partner earns $147,500, pays $38,595, and the household keeps $217,810.
So the couple earns $20,000 less and has $1,394 a month more to spend. The top slice of a single $315,000 income is taxed at 45%, while two incomes of $147,500 each stop $42,500 short of that bracket. Lenders work from take-home pay rather than gross, so the couple's tax saving counts towards what they can borrow.
Worth knowing
How evenly you split it matters. The same $295,000 earned as $255,000 and $40,000 takes home $17,101 a month. That's about $1,050 a month less than an even split, because the larger income runs into the top bracket. The more lopsided the split, the closer the household sits to the $315,000 a single earner needs.