diff --git a/java-compiler-testing/src/main/java/io/github/ascopes/jct/containers/OutputContainerGroup.java b/java-compiler-testing/src/main/java/io/github/ascopes/jct/containers/OutputContainerGroup.java
index 5ced1461a..08faa4392 100644
--- a/java-compiler-testing/src/main/java/io/github/ascopes/jct/containers/OutputContainerGroup.java
+++ b/java-compiler-testing/src/main/java/io/github/ascopes/jct/containers/OutputContainerGroup.java
@@ -15,6 +15,8 @@
  */
 package io.github.ascopes.jct.containers;
 
+import io.github.ascopes.jct.workspaces.PathRoot;
+import java.util.List;
 import javax.tools.JavaFileManager.Location;
 import org.apiguardian.api.API;
 import org.apiguardian.api.API.Status;
@@ -29,12 +31,41 @@
  * {@link #getOrCreateModule(String) create} the module, and then operate on that sub-container
  * group. Operations on non-module packages should operate on this container group directly.
  *
+ * <p>Note that each container group will usually only support one package container group
+ * in the outputs. This is due to the JSR-199 API not providing a method for specifying which
+ * output location to write files to for legacy-style packages.
+ *
  * @author Ashley Scopes
  * @since 0.0.1
  */
 @API(since = "0.0.1", status = Status.STABLE)
 public interface OutputContainerGroup extends PackageContainerGroup, ModuleContainerGroup {
 
+  /**
+   * {@inheritDoc}
+   *
+   * <p>Note that this implementation will only ever allow a single container in the package
+   * container groups. If a container is already present, then an exception will be thrown.
+   *
+   * @param path the path to add.
+   * @throws IllegalStateException if a package already exists in this location.
+   */
+  @Override
+  void addPackage(PathRoot path);
+
+
+  /**
+   * {@inheritDoc}
+   *
+   * <p>Note that this implementation will only ever allow a single container in the package
+   * container groups. If a container is already present, then an exception will be thrown.
+   *
+   * @param container the container to add.
+   * @throws IllegalStateException if a package already exists in this location.
+   */
+  @Override
+  void addPackage(Container container);
+
   /**
    * Get the output-oriented location.
    *
@@ -42,4 +73,16 @@ public interface OutputContainerGroup extends PackageContainerGroup, ModuleConta
    */
   @Override
   Location getLocation();
+
+  /**
+   * {@inheritDoc}
+   *
+   * <p>Note that this implementation will only ever return one container in the list due to
+   * JSR-199 limitations.
+   *
+   * @return the list containing the package container, if it exists. Otherwise, an empty list is
+   * returned instead.
+   */
+  @Override
+  List<Container> getPackages();
 }
diff --git a/java-compiler-testing/src/main/java/io/github/ascopes/jct/containers/impl/OutputContainerGroupImpl.java b/java-compiler-testing/src/main/java/io/github/ascopes/jct/containers/impl/OutputContainerGroupImpl.java
index 3724089e1..33479ea8b 100644
--- a/java-compiler-testing/src/main/java/io/github/ascopes/jct/containers/impl/OutputContainerGroupImpl.java
+++ b/java-compiler-testing/src/main/java/io/github/ascopes/jct/containers/impl/OutputContainerGroupImpl.java
@@ -81,6 +81,24 @@ public OutputContainerGroupImpl(Location location, String release) {
     }
   }
 
+  @Override
+  public void addPackage(PathRoot path) {
+    if (super.isEmpty()) {
+      super.addPackage(path);
+    } else {
+      throw packageAlreadySpecified(path);
+    }
+  }
+
+  @Override
+  public void addPackage(Container container) {
+    if (super.isEmpty()) {
+      super.addPackage(container);
+    } else {
+      throw packageAlreadySpecified(container.getPathRoot());
+    }
+  }
+
   @Override
   public void addModule(String module, Container container) {
     getOrCreateModule(module).addPackage(container);
@@ -173,4 +191,13 @@ private PackageContainerGroup newPackageGroup(ModuleLocation moduleLocation) {
     group.addPackage(pathWrapper);
     return group;
   }
+
+  @SuppressWarnings("resource")
+  private IllegalStateException packageAlreadySpecified(PathRoot newPathRoot) {
+    var existingPathRoot = getPackages().iterator().next().getPathRoot();
+    return new IllegalStateException(
+        "Cannot add a new package (" + newPathRoot + ") to this output container group because " +
+            "a package has already been specified (" + existingPathRoot + ")"
+    );
+  }
 }