Testing is incredibly expensive. If subjects call for the "B" sample to be tested and results return the same verdict they should have to pay for the "B" sample test.
According to this post it isn't expensive at all.
Fees | Australian Sports Anti-doping Authority - ASADA
These charges are valid for all services provided from the 1 January 2020. All charges are per test unless otherwise stated, and are inclusive of GST. We may negotiate revised unit costs where clients enter into contracts for significant testing programs involving combined urine and blood...www.asada.gov.au
If I'm reading that correctly it is $1,000.00 to have a urine sample collected and tested. That's nothing. So it would cost the NRL $500K to sample and test every player in the 30 man squad of all 16 teams. If they did this randomly during the pre season then you can clean up the sport. You wouldn't be relying on tip offs.
Players are more likely to want to "bulk up" and/or work on rehab for off season surgery or injury recovery during the pre season period. That seems the most obvious time that banned substances might be used. If every player knew they were going to be tested between October and March then they are less likely to be tempted. I wonder how many players get tested in a season as part of the random process ??