If they pay late or by instalments, interest accrues on the unpaid debt. 

In some circumstances, taxpayers may be eligible for interest-free payment plans for activity statement debts (interested taxpayers should phone 13 11 42 to find out if they are eligible).
If they pay late or by instalments, interest accrues on the unpaid debt. In some circumstances, taxpayers may be eligible for interest-free payment plans for activity statement debts (interested taxpayers should phone 13 11 42 to find out if they are eligible).

If the tax debt is more than $100,000, the taxpayer can also phone the ATO on 13 11 42. 

Note that taxpayers with a payment plan still need to lodge all of their ongoing activity statements and tax returns on time. 

Ref: ATO's Small business newsroom " Have a tax debt $100K or less?", 19 June 2017