Implicit in the numbers mentioned by Cater are the broader economic variables of the time periods they relate to. So, the $154 bn is in the context of an economy growing below trend and unemployment of around 6% etc. The $270 bn must have been derived with at least an implicit assumption of the level of these, and other, parameters.
Your point about population (and demographic) growth is taken. However, in part-defense of Cater, $154 bn in the context of 6% unemployment is not just “about” nominal GDP growth to a $270 bn figure if long-run unemployment is assumed to be 5%. That is, unemployment is expected to come down over that period, therefore the population of people who receive the payments is smaller. I would expect unemployment payments to grow slower than nominal GDP, considering this.
Also, compounding growth rates of over 5.5% is likely higher than any reasonable forecast/projection of Australian NGDP growth rates over the next decade.