You want to do some hiking while on your dream trip to Australia, but you don't have time for long treks.  Here are some suggestions on short day hikes that may be on interest.

Lake St. Clair - Tasmania
There are three short walks at Lake St. Clair.  These walks can be combined to extend the 2.4KM hike to 4.7KM.
Watersmeet is a short easy walk which will take about 45 minutes.  The track follows an old logging road.
Larmairremener tabelti is a cultural heritage walk which uses creative interpretation panels to introduce the indigenous people of the region.  The track follows moraines, which are ridges formed by retreating glaciers during the ice ages.
Platypus Bay is a 30 minute walk from Watersmeet Bridge.  The track follows the Cuvier River to its mouth at the lake.  The track then curves around the edge of the lake.  Early morning or late afternoon platypus are sometimes seen in this area (thus the name).

Rosewood Creek Circuit - New South Wales
This trek is a peaceful 7.7KM loop walk which loops through astonishing World Heritage-listed rainforest, waterfalls, creeks and wildlife.  beginning and ending at Never Never picnic area, it offers a moderate walk for a leisurely afternoon.  The rainforests in this part of the world are just how scientists believe the supercontinent Gondwana world have looked - in a time when dinosaurs ruled.  The circuit is located near Thora, New South Wales.

Kings Canyon Walk - Northern Territory
The Canyon Walk is known as the spellbinding rim walk.  The 6KM circuit will take you down into the Garden of Eden and back to the top.  While the start of the walk is the most difficult, once at the top, it is the most magical experience of your life.  It is one of the great walks of the outback and something you won't forget in a hurry.  Because the hike is in the Red Centre, there is very little shade and the best time to do this hike would be May through September.

Long or short hikes in New Zealand are the best way to see beautiful landscapes and explore vast wilderness areas. So put on your walking boots, grab a snack and water and set out.
New Zealand is packed with short hikes suitable for all levels of fitness.  Explore native forest trails and ancient glaciers with hikes of just 30 minutes up to 3 hours.  New Zealand short walks cover everything from a slightly more challenging mountain climb to casual beach strolls.

Starting in the middle of Rotorua from beside the Polynesian pools, this hiking trail will take you along the lake’s edge towards the Wakarewarew Forest Park. Along the way, you will pass through the native scrub, across boardwalks in steamy thermal areas and takes you southeast along small beaches. There is informative signage provided so visitors can understand some of the finer details of the natural wonders surrounding you.  The walk is about 1 1/2 hours roundtrip.  There are several accommodations close to the lake and many people choose rooms with the lake view so they can enjoy the beauty of the area even when inside.

For a moderate challenge, test your stamina on the Queenstown Hill Loop Track.  The hike starts from downtown Queenstown up to the Basket of Dreams, which is a favorite lookout spot.  The hike up is a demanding uphill wall ending with fantastic views of Lake Wakatipu, the Remarkables, and other mountains.  On the upward walk, the path eventually turns to the right and you will see a gate.  In the neighborhood, you will see a plaque which reads –
This pathway leads to our future. With each step, we seek the guidance and wisdom of those who have gone before us; we walk with a sense of hope, that those who follow in our footsteps beyond the year 2000 can do so with the same sense of pride in, and protection for, this beautiful place.
As the name indicates, the 1.5km long, 3-hour Queenstown Hill Time Walk is built to show the past, present and future of Queenstown and the area around Lake Wakatipu as informational panels guide you along the way.

This is a meandering and pleasant hike that follows the coast and leads to a seal colony. Informative panels will share the history and geology of the area.  Additionally, there are photographs both old and new to enhance the experience. If you choose, you can take the path from the seal colony up to the clifftop where the view includes hundreds of seals. As someone who has walked on the beach with seals, I must warn visitors to give them a wide berth.
To fully explore and enjoy the features of the hike, allow at least 3 hours to complete the whole Walkway, which extends 11.7 km.

Ironically, in a town built on whales, the peninsula when viewed from above, resembles the fluke of a whale’s tail.