The latest outbreak which plunged Melbourne into a two-week lockdown has served as a reminder of the threat coronavirus poses the community.

It emerged four Victorians who tested positive after travelling through NSW holiday destinations were not linked to the broader Melbourne cluster and have the Delta variant of COVID-19, which Victoria's chief health officer Brett Sutton said was of "significant concern".

Epidemiologist Meru Sheel from the Australian National University said without wide uptake of the vaccine, Australians were "exposed and vulnerable" to the virus.

As the rollout has started to be expanded across some states and territories, thousands more people are now eligible for vaccines.

There are currently only two vaccines available in Australia - AstraZeneca and Pfizer.

Both vaccinations require two doses. AstraZeneca is given 12 weeks apart and Pfizer is administered three weeks apart.

Research has shown the vaccines provide optimal protection from the virus about two to three weeks after the second dose was administered.

It is not yet known how long the vaccine can protect against COVID-19.

The AstraZeneca vaccine has been linked with a very rare side effect called 'thrombosis with thrombocytopenia syndrome', which causes blood clotting.

Australia's vaccine taskforce, the Australian Technical Advisory Group on Immunisation, calculates the risk of getting the syndrome at about 2.6 cases per 100,00 doses in people under 50 and 1.6 cases per 100,000 doses in people over 50.

ATAGI has advised people under 50 are preferred to get the Pfizer vaccine rather than AstraZeneca as a result.

Under-50s can still get either vaccine and should discuss the possible risks and benefits with their doctor.

Australia has agreements to get 40 million doses of Pfizer this year, but 20 million won't be available until at least September.

There is also an agreement for 3.8 million doses of AstraZeneca to be supplied from overseas. A further 50 million doses will be manufactured in Victoria.

The Australian government has entered into agreements for two other vaccines which haven't yet been approved by the Therapeutic Goods Administration - Novavax and Moderna.

Novavax is currently being evaluated by the TGA. The federal government expected 51 million doses would be available if it was approved later this year.

Moderna is currently in phase three clinical trials.

The federal government expected 25 million doses of the vaccine would be available including 15 million doses of a "variant-specific" version, should it be approved.

Vaccinating the population against COVID-19 will protect against big outbreaks of the virus and reduce the risk of severe disease for those who have had the jab, Dr Sheel said.

"It's going to prevent resurgence of the disease. Australia has done incredibly well but as we've seen, outbreaks can happen really quickly," she said.

"We can again see how quickly there can be a resurgence even in island countries despite having closed borders."

Dr Sheel said anyone who was hesitant about receiving either Pfizer or AstraZeneca, particularly regarding the risk of the rare clotting syndrome associated with AstraZeneca, should seek advice from medical experts to talk through their situation.

She said people who were eligible should book in rather than wait to get a specific vaccine.

Supply issues have plagued the rollout so far. Dr Sheel said it wasn't known when more Pfizer doses would be available, nor what the global situation would be at that time.

"We know that if you were to get Covid, you could have a 10 times higher risk of clotting, but also the risk of long Covid is quite significant," she said.

"It's important not to wait because we don't know when Pfizer vaccine will become available ... in that time, people are exposing themselves, they as an individual are vulnerable to the infection, but also their family and friends are exposed and vulnerable."

Most jurisdictions require residents to book in for a vaccine at either a general practitioner or a dedicated COVID-19 vaccine clinic.

Vaccination is free and voluntary for everyone.

Once the vaccine is administered you must remain at the clinic for observation for at least 15 minutes.

Depending on your medical history you might be asked to wait longer, about 30 minutes.

Across Australia everyone over 50 is eligible for the vaccine. Under-50s who are in phase 1a and 1b are also eligible.

This includes health, quarantine and border workers, people with disabilities and people with underlying medical conditions.

Some states have expanded eligibility to younger Australians.

Eligible groups:

NSW: Over-40s and under-50s in phase 1a and 1b.

ACT: Over-40s and under-50s in phase 1a and 1b.

SA: Over-16s in regional areas, all Aboriginal and Torres Strait Islander people over 16 and over-50s in all other areas.

WA: Over-50s and under-50s in phase 1a and 1b.

NT: Over-16s in regional areas and, in greater Darwin all people over 50 and under-50s in phase 1a and 1b.

QLD: People 40-49 can register interest. Over-50s and under-50s in phase 1a and 1b.

TAS: Over-50s and under-50s in phase 1a and 1b.

VIC: Over-40s and under-50s in phase 1a and 1b. Expanded list of vulnerable groups.