One day visit from Hong kong (Catch train in Hong kong. in & out same day)

Hi
Will be in Hong Kong for 6 days.
We have Australian passports & UK Passport. (Dual citizen)
Not sure but MIGHT go to China for day trip.
Do we need Visa for less than 24 hours?
Or if still need it,
Will we be able to get it while in hong kong on the day & if so will it be more expensive
or must get before leaving Australia?
Regards
Harry