Singtel-Optus is increasing its data allowances on two high-end plans in readiness for the likely launch of Apple's iPhone 6s on September 9 in a move that analysts say will crank up pressure on Telstra to boost its offers.

From Wednesday Optus will give customers who are on $100 a month contracts a 12-gigabyte download allowance and 400 minutes of overseas calls. Users spending $135 a month will also have their data caps raised to 20GB – matching a similar offer to Vodafone Hutchison Australia's.