Sydney Domestic to International Transfer Time

My partner and I are travelling from Brisbane to Johannesburg, via Sydney. We have booked directly through Qantas for both legs. Our first flight lands in Sydney at 8am, with our Sydney to Johannesburg flight departing at 9:30am. So if everything goes to plan (which I know is not always the case), there is a 1.5hr connection time, which is Qantas MCT. Our check-in luggage will be transferred between flights for us, so no need to collect and re-check it in. For those who have done the Sydney domestic to International transfer, should this be enough time or will we be cutting it close? Just trying to figure out if we need a slightly earlier flight to make sure we get to international with enough time. Hoping to avoid that as to change to earlier flight will cost just over $1000! Thanks!