They're not withholding the tips, they're just handing everything to administrators who will deal with the debts of the company as must be done under the law. The alternative would prejudice any secured creditors, who are afforded preferential treatment in the winding up of a company and its assets. Most companies that enter administration have (at best) a small amount of cash, a large amount of debt, and little revenue. If you are still solvent and trading you can pay off your debts in the order you choose, like paying your drivers what you owe them as those debts arise even if you have to restructure larger debts (on the basis of your prospective revenue) to do it; if you cease to be solvent or cease to trade, you must pay them off according to the rules of winding-up.

Given that it's voluntary administration and not forced, I would expect a lot of the regular creditors (including drivers) to be paid to some degree, as Deliveroo Australia probably still has some cash. But they can't just predict that to be the case and distribute the company's assets on that expectation. Administration means handing it all over and wiping your hands clean. Whatever's there will make it to whoever is owed money if the company has the assets.

Maybe, but your lens might be off here. Tips aren't a fundamental part of Australian culture, and no driver (or anyone in hospo) is reliant on them like some other countries.