With reference to Tiger even if you can book eg a Darwin/Singapore/Phuket trip in the one booking your luggage is only checked as far as the first destination ie Singapore. So irrespective whether the flights are in the one booking or not if you have excess baggage you will need to pay for this each time upon check-in.
It clearly states in the conditions before your reach the end of the booking process that the baggage allowance is 15 kilos and excess baggage rates apply after that so no-one should use the excuse "nobody told me I could only take 15 kilos".
If you know you are going to have more than that put simply your options are: a) prepay excess at the time of booking - this cannot be done at a later date b) get a quote from Pack & Send or some other freight company c) pay excess baggage at the airport however many times you need to recheck in.
Then there's always the option of sticking to the limit or being really bold and taking carry on only - ooh aah!
One bloke I heard about flew BKK/SIN/DRW shelled out about 600 bucks and literally had to sell the book he was reading in-flight so he could buy some potato chips and a drink on the SIN/DRW sector coz he was completely out of coupons!!