@TracyA 

Hi,

by "Av1600" do you mean the Archer VR1600v AC1600 DSL router?

Can you tell us the make and model number of your "nbn modem"? Is this a pure modem or a modem router?

Have you configured the "Internet" connection of your new AX3000 router as "Dynamic IP" or "PPPoE"?

@woozle 

yes that's what i meant sorry. 
it's a modem router and is an Arris CM8200B. 

And yes i have configured from Dynamic to PPPoE

 any thoughts?

 Kind Regards 

@TracyA 

Do you have internet access on the computer if you connect it directly to the Arris CM8200B modem? Do you need to dial up on the computer?

If there is no need to dial-up on the computer when connecting to the Arris CM8200B modem directly, please choose Dynamic IP on the AX3000 router when configuring. Check if you can get a valid IP address from the Arris CM8200B modem. You can also try to clone the MAC address of the computer to the AX3000 as follows:

https://www.tp-link.com/support/faq/2264/

BTW, who is your ISP? You can also confirm with them that if you need to have any special setting on the router in order to get connected from the router.

@Kevin_Z 

I already called TPG and they spent a great deal of time trying to get the router and modem to sync together. Technical support seemed to think it's the modem. 
I don't need to physically connect my computer to the modem. So will keep configuration as Dynamic then. 
I'll try cloning MAC address and see how that goes.