Under present community rating legislation, VHI or any other other insurer can't charge you more than they charge anyone else, so the break in cover won't be held against you in that regard.
However, you may have to serve waiting periods if you rejoined after a break in cover of more than 13 weeks, i.e. your application is treated as a new application and your previous cover isn't accounted for.
See if there are any Australian health insurers that any of the Irish health insurers recognise, and you may be able to get credit for your time abroad under that health insurer. However, I don't think this happens anymore, but please let us know if you find out otherwise.
Do you have any idea yet of when, if at all, you would be returning to Ireland? If it's definitely in the next year or so, then it may be better to pay VHI Global and have that peace of mind when you return. If it's not for the forseeable future though, then maybe put the money you're saving aside as an emergency fund should something happen if you return.
FYI, the three waiting periods are:
Initial waiting period - 26 weeks for people under the age of 55 when joining. This basically means that in-patient or day-case treatment isn't covered for the first 26 weeks, unless its for an accident that occurs after you rejoin. If you're 55 to 65, then it's 52 weeks, and 65 plus is 104 weeks.
Maternity waiting period of 52 weeks - In-patient or day-case treatment relating to pregnancy isn't covered for the first year of the policy.
Pre existing waiting period - 5 years for people under 55. This means that in-patient or day-case treatment relating to any illness or condition that developed before rejoining won't be covered for the first 5 years of the new policy.