We will fly SYD/CAN June and then CAN/BKK 12 June. Can we apply for 144 hour tourist transit visa?

Yes, you can. Your itinerary is definitely eligible for 144 hours visa free transit in Guangdong province. You should keep in mind that your both SYD/CAN and CAN/BKK flights should not have any additional landings within mainland China. By the way, please, use correct terminology because you will not apply for "tourist transit visa", but for visa free transit (without any kind of visa) to avoid any possible misunderstandings.