There is no way of knowing how many illegal immigrants there are.
If you're referring to asylum seekers who arrive by boat, as soon as they apply for asylum, they're no longer considered as illegal immigrants, they're legal asylum seekers.
Any attempt at estimating the number of illegal asylum seekers is automatically ridden with guestimates.
If the government knew of illegal immigrants, they would arrest them.
The illegal immigrants consist of visa overstayers, (students, tourists, temporary workers, etc), who arrived on standard transport, and didn't leave by the end date of their visa.. Some of them may have already returned to their own country, the government does not know.