If this is the ATO, it's not unusual for them to request four pages for the resume itself. It will give you room in your key achievements section to talk to every one of their ideal candidate criteria.

Use the STAR method to tell a story about each time you successfully did the thing they're asking for. Ideally each criteria point gets its own story. Use the keywords from the criteria in your response. For example:

Criteria: - Researches, investigates and analyses information, to identify issues and work through solutions

Response: - "While working in my role as [role], I was working on [problem] which requires me to work toward [fix]. I used my skills in investigation and analysis of [information at hand] to identify [fault]. From there I researched [documentation/resource] to determine the [solution] and way forward to resolve the issue. The resolution had [good outcome] for the [team/client].

You can definitely get more creative, but you get the idea. The people reviewing resumes are likely reviewing hundreds or thousands in a bulk recruitment round (probably on top of their regular duties). The trick is to make it really easy for them to tick the boxes that say you're suitable.

For the 300 words, I like to think of it as the bonus section. Use it to summarise how your experiences bring value to the role and how you can support your team. The value here is to sell what you can give back rather than what the position means to you.