The Medical Journal of Australia recently reported that of the 4.1 million unvaccinated Australians, 92 per cent (3.8 million) were adults.
There are a number of reasons behind an adult not being vaccinated — maybe their parents were worried about side effects, or after one dose they had a reaction.
Associate Professor Nicholas Wood from the National Centre for Immunisation Research and Surveillance (NCIRS) said another factor was Australia's "luck".
"Some of these diseases are not as common as they are in other parts of the world, and so people say, 'I'm at low risk of getting it, therefore getting vaccinated is not at the top of my priority list'," he said.
So what do you need to know if you decide to catch up on your vaccinations?
First things first, visit your GP
Dr Wood said first up is a visit to your GP, who can put together a catch-up schedule for you.
"That involves several doses of a few vaccines — particularly diphtheria, whooping cough, and tetanus," he said.
The tetanus vaccine requires three doses for a person to have a reliable protection against the disease.
"You need at least one dose of the measles/mumps/rubella vaccine (MMR) — and ideally two," he said.
"You probably also need a chicken pox vaccine — if you get chicken pox as an adult it can be particularly nasty."
As well as the itchy spots produced by the chicken pox disease — which can leave a scar — some people can also go on to develop pneumonia and in rare cases a brain infection such as encephalitis.
Dr Wood said there were certain rules for catch-up schedules, such as a minimum interval between doses.
Some people might decide to get a blood test to check what immunities they may or may not have.
Vaccination-produced immunities, and natural immunities, may show up in a blood test, but that can be largely dependent on how long ago the vaccination was administered.
Dr Wood's advice is if you are unsure, get the vaccination.
What if I had chicken pox, measles as a kid?
The first question for anyone who believes they had any of these diseases as a child, said Dr Wood, was to make sure that it actually was chicken pox or measles.
"There are many viruses that can cause a measles-like rash," he said
The only way to be sure would be if there was a laboratory confirmation of the disease at the time the person had the disease, and it was detected.
So if you only know that your mother thinks it was chicken pox or measles, but never had it tested to double check, then you're better off getting the vaccine.
"Because all it does is boost the immunity that you got from the natural infection," Dr Wood said.
Would there be any issue with someone getting a vaccination because they were uncertain, but in truth had one of those diseases as a child?
Dr Wood said no, it is still safe to get the vaccine.
In fact, even if someone had a disease like measles or chicken pox as child, it would still be a good idea to get the MMR vaccine to protect them from the other diseases covered in the vaccine — mumps and rubella (German measles).
Over 15 cases of measles were recently detected in Sydney, bringing the total in New South Wales this year to 22.
How do vaccinations work?
People often cite the benefits of having a natural immunity to diseases, which only happens if a person has the live disease at some point in their life.
But Dr Wood said while that was considered "ideal", there was no way to ensure that person only experienced the mild form of a disease.
"Because the way each person's body deals with the disease is different and we can't predict what you will do if you get exposed to the disease," he said.
"You might be lucky and get a mild disease. Or you might be unlucky and get the really nasty severe complications of the disease."
Dr Wood said the vaccinations being used have been around for decades, and there was a long history of understanding how they worked and how safe they were.
"Adults should be reassured that we're not experimenting, we're giving them tried and tested vaccines and our aim is to prevent them from getting the nasty real-life diseases," he said.
The way a vaccination works is by mimicking the disease without actually giving the person the disease.
"So the vaccine in a sense tricks the body into thinking that they're seeing the real disease," he said.
"It then makes the antibodies, and the cellular immune response.
"So if down the track you see the real disease, your body goes, 'Ah I recognise this, I'll know what to do' — so you don't get the full blown disease."
How much will this cost?
The Government will cover an individual's vaccination catch-up until the age of 20.
From there it depends on the person's situation in terms of whether they are pregnant, and whether they work in an environment considered "risky".
"If you're a pregnant woman you can get the influenza and whooping cough vaccine as part of the free program in most states of Australia," Dr Wood said.
Healthcare workers, people in the armed forces, police, people working in hospital laboratories, sex workers and others working in similar environments, can get vaccines such as Hepatitis B for free.
But if you are not included in any of those categories, then the cost will fall on you.
The vaccination for poliomyelitis (polio) is given to children in a combination vaccine protecting them from altogether five diseases — polio, diphtheria, tetanus and whooping cough.
But if someone did not receive that childhood vaccination, then the cost would be roughly $120 — the vaccine, Ipol Polio, can be found at most chemists.
The vaccine for polio is administered over three-doses, with a break between each dose — each one costing around $40.
Some GPs will have the vaccine, however, often the price will be more affordable if bought from a chemist and then taken to the GP or nurse to be injected.
"Polio is not really an issue for Australia in terms of local transmission, but it certainly still exists in the world — we haven't eradicated it," Dr Wood said.
And he said an adult exposed to polio would be more likely to end up with a more severe version of the disease.
But wait, there's more
Once you catch up, you should keep in mind the potential need for booster shots later on down the track.
If someone is travelling to a country that is known to have a disease such as polio, then it is recommended that they consult their GP about a booster shot.
The same goes for those workers in "risky work environments" — they may need booster shots throughout their lives and sometimes blood tests to ensure the vaccines are working.
"So in general things we like to boost [are for vaccines] like tetanus and whooping cough," Dr Wood said.
"But other ones like Hepatitis B and MMR — once you've had your primary course, then you're ok."