You can only claim the business use percentage if an expense is only partly for business purposes.

You can’t claim a deduction for an expense that is solely for private use. If an expense relates to both business and private use, you have to work out what proportion of the expense is related to your business and claim only that amount. If you’re  registered for GST, you will also apportion your GST credit.

If you purchase goods or services for both business and private use, you can only claim a GST credit for the part of the purchase relating to your business use.

There is no limit on the amount that you can claim each year, provided the expenses were incurred when you were earning your assessable income. You need receipts or other documents to substantiate the expenditure.