Tigerair Australia already has a deserved reputation for meanness with baggage: it charges a fortune for excess baggage and weighs your hand luggage before you board to make sure you comply with its limits. That process will now be even harder, since the allowed weight for carry-on has dropped from 10kg to 7kg.
Tigerair's switch means that all four major domestic airlines now have a 7kg limit, spread across a maximum of two items. The partial exception is Qantas, which has a weight limit of 7kg per item, and allows you to take two items on board unless you're travelling on a Dash-8.
If you want to carry more than 7kg on Tigerair and don't want to check your baggage, you can pay for the so-called Cabin+ product. That raises your carry-on allowance to 12kg, though you can still only have two items. Cabin+ costs $18 (for shorter flights) or $23 (for longer flights) if purchased in advance; those prices jump to $36 and $46 if you pay at the airport.
$36 is a better deal than the minimum $70 you'll pay if Tigerair asks you to stow your bag in the hold and you haven't paid in advance. But it's still expensive — if you're on a discounted ticket, it could well be more than the actual fare.
The new rules will apply for all flights booked from today with a flight date of 17 April or beyond. If you've already booked a flight for a post 17 April date, the old 10kg limit still applies — though arguing that point with a gate agent is likely to be troublesome.
The lesson, as always, is to be clear what the rules are, make sure you stick within those limits, and remember to include luggage costs when you're calculating the total fare. "Low-cost" airlines can cost you more in the long run — especially because in reality, if your carry-on bag fits the size requirements, neither Qantas or Virgin is at all likely to check the weight.