Here are the carry-on baggage rules per the Jetstar website

Carry-on baggage – what can I bring on board?

Jetstar fare/Carry-on baggage allowance
Starter
Starter Plus
Starter Max

• 1 main item per person, plus
• 1 small item

Combined weight of 7kg

FlexiBiz bundle

• 1 main item per person, plus
• 1 small item

Combined weight of 10kg

Business Class

• 2 main items per person, each 7kg or less, plus
• 1 small item

Combined weight of 14kg

Carry-on baggage size limits
Your carry-on bag (or bags in Business Class) must fit in the overhead lockers and be no bigger than:
56cm (height) x 36cm (width) x 23cm (depth)

Your small item must fit under the seat in front of you.
Small items
The small item you bring on board could be:

• A handbag, a pocket book or purse
• A laptop computer in thin case
• An umbrella or walking stick
• A small camera
• Infant food for consumption in flight
• Approved duty free goods