7 kg for hand luggage is awfully tight.
7kg is awfully normal in Europe and Australia. However, I've never had my hand luggage weighed with Qantas.
That said, it doesn't look like it exceeds 7kg so they have had no reason to.
For a laptop computer to be considered a personal item it must be carried in a slim bag. Laptops carried in large laptop bags will be counted as part of your carry-on baggage allowance.
This is because they allow an extra cabin bag if it's a small laptop bag - but people have been exploiting the rule by stuffing backpacks with clothing and other stuff until it resembles hold luggage. Generally speaking, Qantas staff are easy-going if you're sticking within the limits more or less, but you do get the odd stickler who will check things.