I want to drive a delivery van in Australia and I m new in Australia so I don't know the rules for driving a van
Whether you are allowed to drive it without taking a test depends on the weight of the van you will be driving. If it's classed as a heavy vehicle then you will need to complete a theory and practical test. If it is classed as a light vehicle then you can drive on your UK licence for 3 months after which you must change your licence to an Australian licence. Heavy vehicles are usually 4.5 tonnes GVM or more. So, if it's just a van like a Hiace then you will be fine on a car licence; however, if it's a larger van or box truck then you should consider the GVM based on the loads you will carry.
The UK has a reciprocal agreement with Australia which means that you don't need to take a theory or practical test if you move there with a full licence and want to drive a car or ride a motorbike, as long as you have the same class of licence from the UK. However, this isn't the same for commercial heavy vehicles.
This page has good general driving tips for Australia: https://en.wikivoyage.org/wiki/Driving_in_Australia
This page outlines rules and regulations around driving heavy vehicles in Australia: http://www.driverknowledgetests.com/resources/loading-and-driving-heavy-vehicles/
Note that if you have taken a Highway Code LGV/HGV test in the UK that there is some very different knowledge you will need to know in Australia. Also, that the road rules and road markings are quite different in some aspects. It's advisable to study the rules as a delivery driver, especially those around where you are allowed to park.
It is likely that an employer will require you to change to an AU licence because of the 3-month time limit.