As an EU citizen, Macau will give me a 90 day visa upon arrival.
Can I visit Hong Kong with that every other weekend or will leaving Macau void my visa?
According to the official website, entry permits (visa-on-arrival) are only valid for 30 days and you don't need one as an EU citizen. What you'll get is 90 days of visa-free stay.
If you leave Macau, you will need to go through immigration when you come back and you could then theoretically be denied entry.
The same website specifies that
In practice, staying in Hong Kong and travelling to Macau, I got a new entry stamp/authorization to stay each time very easily. I expect things to work the same the other way around but I don't know how often you can do that before raising suspicion.
Yes, you can visit Hong Kong and re-enter. To be exact, as an EU citizen you do not need a visa for Macau, you will be granted a 90-day
Beware that, if you keep doing this, you may eventually be refused entry for
So you'd be advised to get a work/study/extended stay permit from Macau if you have reason to stay there longer, and I presume you do, because even a hardcore gambler would get a wee bit bored after 90 days!