Ok, not to disagree with Wilson (since I don't know if he's saying no to me or to you) but let me put it another way. Right now it's just on the middle of January. You have between now and Trials time to prepare for UMAT, and (according to BoS timer) 272 days before the HSC.
Now, with the UMAT, "studying" in the HSC sense is not only unnecessary, but may even be counter-productive. In fact, as a test designed to require absolutely no prior preparation, ACER expects people to walk in and generate results based on their inate abilities alone. This of course is unrealistic for most people, who fairly engage in some form of preparation (more on that later).
Now consider the HSC. This year-long marathon of study and preparation requires and demands extreme devotion in order to achieve the marks required for consideration by medical schools. There is absolutely no way any sane person can walk into an exam without studying slave hours for a year and even hope to get into medicine (except maybe me, but that's a story for Thursday if I'm lucky). I'm sure if you go to a halfway decent school your teachers urged (perhaps even begged) you to do some work over the summer, because it could make all the difference between 89.00 and 90.00 later on (which, in Adelaide, is the difference between eligible and ineligible - and that's the lowest cutoff in the country). Cutoffs aside, we all know it takes a lot of work to reach the levels expected of medical candidates.
Now let's look at the requirements of UAI and UMAT across the different schools.
In NSW, UNSW determines interview allocation on a "decent" UMAT score (see their website) and projected or real UAI of no less than 98.00 (this part is undisclosed). Unless you get a predicted or real UAI that high, you're not studying med at UNSW. Newcastle is more kind, expecting scaled scores around 180 (mid to high range) with an academic eligibility threshold of around 93.80 - much lower than UNSW, but high nonetheless.
In Victoria, Melbourne university will admit candidates with pathetic UMAT scores (try 50th percentile) so long as they have a high enough ENTER. I can't say much about Monash, but from memory their interview allocation is based on UMAT (like Newcastle). I think they (like UNSW) weigh final selection ranking on an equal basis of UMAT, UAI and interview.
Tasmania, well, they rely on a combination of TER and UMAT for entry, but their course is extremely academic and the TER seems to hold far more weight.
Adelaide, as mentioned before, has the lowest TER threshold at 90.00 - like Newcastle, once you exceed this score your UAI becomes irrelevant. UMAT is used to determine interview allocation, and (again like Newcastle) interview performance determines selection ranking.
UWA selection is similar to UNSW, but with more equal weight on the UMAT and interview (rather than UAI bias like UNSW). Equal combination of TER, UMAT and interview to determine selection ranking.
As you can see, Wilson's comment about the UMAT having a large weight in medical selection is not unfounded. However, if you have a less than glamorous UMAT you can still save the day with a spiffy UAI, especially since you can always sit the UMAT again next year without too much hassle. A rusty UAI, however, will plague you and your sons unto the seventh generation. I know this from experience.
Most certainly get some materials and practice the UMAT if you feel you need to. But under NO circumstances should you put it on equal or higher priority to your HSC work. This year, school ALWAYS comes first.