Hello.
I essentially want to “complete” a number of tasks based on a different task being completed.
So a bit like a task being dependent, but rather than you being able to complete it once the main task is completed, the dependent task automatically completes.
Hi @anon17038006 and welcome to our Community 
Sounds like using Rules could be the best option! You can learn more about Rules in the following guide article: Using Rules in Asana | Product Guide • Asana Product Guide 
Hope this helps, but let me know if you have any follow-up questions!
Hello, I’ve tried to use rules but it seems to apply that rule to every single task rather than specific tasks.
Unless I’m missing something, but I can’t see a way to set up rules for specific tasks within a project 
In rules you could complete a task (action) when something happens (trigger) like “task is not longer waiting”. Looks like that does not make sense in your case. Are you referring to subtasks being completed when the main task is complete?
Hello - ideally it wouldn’t be subtasks as I’d like them to be in the main list view. So just when 1 task is completed a number of other tasks are automatically completed.
Then you can use a rule and dependencies. WHEN task is not longer waiting (dependency has been completed) THEN complete.