You’ve obviously made quite a commitment to your piri-piri chicken and wonder why it is not reciprocating. Is a little heat missing from the romance?
The first serious question is, are you able to get authentic piri-piri peppers in Australia? If not, what are you substituting? Piri-piri peppers are native to Brazil, but were taken to Angola by Portuguese explorers and became such a part of the local cuisine that they eventually came to be known as Angolan peppers. Many Portuguese keep a bottle of piri-piri sauce on the table and sprinkle it liberally on everything from French fries to shellfish.
Following the general rule that the more compact the pepper, the more fire it harbors, piri-piri are tiny and fiery. Until recently, we have not been able to get them in this country, and many recipes we’ve seen that call for them offer hot New Mexico chiles, chile pequins, and other fireballs as substitutes. You can order a piri-piri sauce kit (just add oil) online, from a company in Portugal that ships worldwide. Maybe that will help.
Are the peppers you are using spicy enough? If the relative strength of your peppers is less, increasing the amount will give you more flavor. Are you discarding the peppers’ seeds and ribs before you add it to the oil? Chop the pepper and add all of it, including seeds and ribs, to the marinade. You can strain them out at the end, but they add a lot of the pepper’s flavor. Cutting the chicken in smaller pieces will certainly expose more surface to the sauce and give you more flavor, but be careful that you don’t overcook the smaller pieces.
It is also possible that the restaurant has some advantages that you don’t. Restaurant grills have more horsepower than most home grills. Perhaps they use some particularly aromatic wood chip to add smoke in the barbecue. Finally - a thought we don’t like to entertain - we know some sneaky chefs who hold back details when giving out their recipes - they don’t want you to succeed in recreating their dish back home. Even if you live 12,500 miles away (as you do), they want you to come back to the source and be dazzled again. Is it possible you came upon one of these nefarious Portuguese chefs?