If the NRL want to have an even salary cap to ensure an even competition they have to do something about TPAs. NRL is heading the way of the EPL , their will be the same 4-5 teams in top 4 every year if it continues.
For mine the biggest problem around TPAs is that they allow clubs to break/bend salary cap rules and have a far better roster than the Salary Cap allows.
The NRL can stop this by one of two ways if they wanted to:
1. Ban TPAs and dock points from clubs that are found to break the rules.
2. Allow TPAs but have a players minimum value under the Salary Cap determined by the NRL and you have to fit your top 25 under the NRL Salary Cap. This would work as for example the Broncos had TPAs in place for Darren Lockyer to make $1 million a year but put him in the cap at $300,000. If the NRL determine he has to be put in at $800,000 a year under the cap, that would mean their is $500,000 less cap space for the Broncos for their other players.
Option 2 would work and wouldn't intefer with what an individual player could earn, if would just mean to have good players in your top 25, a lot of your top 25 would have to be lesser players. If this was brought in many squads would look worse than they currently are but the talent would be more evenly spread (which should be the NRL aim).
Option 2 would still allow players to earn more than what they are assessed at under the cap by way of other TPAs but would have them placed in the salary cap at the right money.
The NFL do this best, they have a hard salary cap. They have rules about minimum amounts players have to be paid under the cap according to their status. The NFL want a team to have a small window at the top and then go through a rebuilding period and be replaced by other teams at the top. They do not want the same 3-4 teams at the top every year. The salary cap in the NFL does not stop the top players making other money outside their club from sponsers but it does stop teams stockpiling all the best talent every year.
