If you crunch a few figures in the NZRB Annual Reports from 2004 to 2017, and compare total costs, as a ratio of NBR(net betting revenue), then
2004 NBR 177.7 mil
Costs 108 mil
Ration 60.8%
This figure stays reasonably constant through till 2010(63.6%, 64.7, 56.3, 60.6, 63.1, 61.2), then in 2011 it starts to climb
2011 64.3%
2012 68.6
2013 65.9
2014 68.6
2015 71.9
2016 73.0
2017 73.8
I've used NBR as a measure, as I think it better reflects the performance of the TAB, as opposed to total turnover
And if you extrapolate out the 2018 6 month report, then NBR is tracking towards 302.1 mil, with costs at 218.2 mil, a ratio of 72.2%