When the journey includes North America or some Central/South American countries, the entire itinerary is based on the piece allocation system. Under the piece allocation system, you are entitled to two checked pieces as a basic starting point, and additional pieces are allowed by some airlines based on FF status or cabin purchased. These additional piece allowances vary by airline.
Under the piece system, each airline is free to determine the maximum weight they will permit for each piece. Qantas recently reduced this from 32kg to 23kg. AA and BA have limited to 23kg for some time now.
When checking-in for multiple connecting sectors using different airlines, you are generally limited to the per-piece weight of the first airline of the connecting flights and they will also generally allow any FF status based additional pieces to be checked through as well. But note that subsequent check-ins may be limited to the per-piece weight limit and FF status additional allowances for the check-in airline and not your home FF airline, the first airline in the itinerary or the ticket issuing airline. Its based on the check-in airline.
However, some people have been able to successfully convince check-in agents to allow per-piece weights in excess of the check-in airline's limit based on higher limits set by other airlines in the itinerary. But that is not guaranteed or a defined policy, so don't expect that it will always happen.
If your journey uses the piece system the ticket should show "2PC" or similar under the Allowance field.
If the journey does not include countries that use the piece system, then you are bound by the weight system. Qantas recently changed their weight system limits to align more closely with their new piece system per-piece limit and with other airlines. You are generally limited to 23kg as non-status economy passenger, and additional allowances go up from there for FF status and cabin purchased. Note that for Qantas, no single piece should exceed 23kg, so if you are a Gold FF member and have an additional 10Kg allowance and want to take say 30Kg you will need to have that distributed between at least two checked bags.
On the weight system, you are limited to the weight allowance of each carrying airline and this may vary between sectors. Similarly to the piece system, if checking-in for a multi-carrier journey involving connecting transits you will likely be allowed the allowance of the check-in carrier for the entire connecting journey. So if a Qantas Club member and hence having a total allowance of 33kg in economy and checking-in with Qantas and then having a connection to Cathay Pacific, you will likely be able to check the 33kg to the CX destination. But on return when checking-in with CX you will be limited to 23kg or whatever the CX economy weight limit is for the flight. To take advantage of the Qantas Club additional limit on the return journey you may have to check the bags only to HKG and then re-check with Qantas for the last segment, and may still be charges excess baggage by CX for the journey on CX to HKG.
