Pardon my ignorance but is it obliged by law to have monarchs on one side ?
That is in the empire where the sun never sets 
The Empire is long gone, and no uniform rule or regulation now applies throughout the Commonwealth in this regard. Each independent country where Elizabeth II is sovereign would have different rules on whether or not the monarch's portrait had to be on the coinage, and which design of portrait is to be used.
Australia, for instance, does have this requirement. It's not enshrined in Law -
Section 13A of the Currency Act of 1965 simply states that the Treasurer shall decide on the coinage design. But the Treasurer back in 1965 left strict guidelines to the bureaucrats in charge of actually implementing the law that all approved designs had to have the monarch's portrait on the obverse, and no Treasurer since then has been sufficiently anti-monarchist to change that.
On the other hand, Papua New Guinea is constitutionally almost identical to Australia (an independent constitutional monarchy with Elizabeth II as sovereign) but almost never uses a portrait of the Queen on it's coinage, unless the coin is actually commemorating a royal event.
Back to the Topic of the heart-shaped coin.... um, yeah.
Will somebody
please tell the Perth Mint marketing department to quit the gimmicks?
