You will find it tough without a sounder as the reef is a large area of low foul with the fish often in patches and not all over the reef.
A good place to start would be at the fairway buoy (the furthest one out - red and white) and head due south from there. Once you have gone about 800m you should start hitting the foul. It runs right across Bream Bay from there right down to the Waipu end. If you can locate a chart you will see where I mean.
There should be a few snapper out there at this time of year and probably more than a few boats on a good day so this would help you out.
Best of luck.